Life on Earth developed in water and, despite evolution over many millions of years, today—as then—all living processes, with their underlying biochemical reactions, are possible only in an aqueous milieu (Chap. 6).

10.1 Water as an Environmental Factor

10.1.1 Water Use by Plants and Animals

Organisms in their active state do not tolerate desiccation. This is illustrated by the water concentration of tissues: in the active state, the protoplasm of leaves and fleshy fruits contains 0.85–0.90 g H₂O g⁻¹ _FW (fresh weight). The water concentration of wood decreases to about 0.50 g H₂O g⁻¹ _FW due to the high contribution of structural carbohydrates and polymers in the xylem. The lowest values are reached in dormant seeds: 0.05–0.15 g H₂O g⁻¹ _FW.

Land plants must keep the water concentration of their cells close to saturation or be fully saturated in an environment of relatively dry soil or air, and still maintain exchange of CO₂ with the atmosphere for photosynthesis (Chap. 12). Terrestrial life outside water brings benefits as well as dangers for plants, for the following reasons (Cowan 1977):

The diffusion coefficient of CO ₂ in air is about 0.14 × 10⁻⁴ m² s⁻¹ and decreases in water to 0.16 × 10⁻⁸ m² s⁻¹. Thus, CO₂ diffuses 10,000 times faster in air than in water (Sestak et al. 1971). In the Lower Devonian, when land plants evolved, the CO₂ concentration was significantly higher (about 4000 parts per million (ppm)) than it is today (about 400 ppm), which would have made the atmosphere even more attractive for plants in the past.
During the evolution of plants, no membrane has been “invented” that is permeable to CO₂ but remains impermeable to H₂O vapour. Even in the future, there will not be a type of “GoreTex” for CO₂, because the molecular weight of CO₂ is larger than that of H₂O (44 versus 18). Of course, it was the availability of CO₂ in the atmosphere as a resource that made plants adapt from life in water (algae) to life on land. However, in order to use this carbon supply, mechanisms had to be developed to regulate the cellular water relations of land plants.
Water relations are more important for the gas exchange of land plants than for animals because of the chemical composition of air. Photosynthesis creates a CO₂ gradient of about 100 ppm between the atmosphere and the mesophyll. At the same time, there is a gradient of water vapour of about 12,000 ppm between the water-saturated mesophyll walls and the ambient air. CO₂ diffuses 1.6 times more slowly than H₂O vapour, as the diffusion rate is related to the square root of the molecular weights

$\left({CO}_244,{\mathrm{H}}_2\mathrm{O}\ 18,\mathrm{and}\ \sqrt{\frac{44}{18}}=1.6\right)$

. Thus, during uptake of 1 mole of CO₂, the plant loses about 200 moles of H₂O

. Therefore, water use (here, water loss) by plants is very high relative to the photosynthetic gain.

Water use by animals differs from its use by plants. For mammals (warm-blooded animals with a body temperature of 37 °C), with 210,000 ppm O₂ in the atmosphere and 160,000 ppm O₂ in the breath, the O₂ gradient is about 50,000 ppm. The air that is breathed out is water saturated. Therefore, the water vapour gradient between the lung and the atmosphere is also about 50,000 ppm (at 20 °C, 50% rel. air humidity). Thus, a warm-blooded animal loses only about 1 mole of H₂O per mole of O₂ taken up. For cold-blooded animals the water loss related to O₂ uptake is even lower (about 0.2)—that is, water use related to O₂ gain is very low in animals. In addition, animals can move to a water source or protect themselves from adverse conditions, giving animals another advantage over plants under dry conditions.

As the loss of water from plants is so large, it is usually not cost effective for them to have water storage. During the course of a day, a sunflower leaf loses about ten times its own weight as water vapour. A 25 m high spruce loses about 100 – 1000 L of water per day, which is more than there is in the stem. The available water stored in the stem and the crown of spruce is sufficient to maintain transpiration for only about 2 h in the humid morning hours (Schulze et al. 1985). This means that the costs of providing water from storage sufficient to safeguard the supply over days or months would be unreasonable: plants would have to construct enormous water stores. The aforementioned example of the daily water loss by a spruce tree shows that a 10-day drought period without water uptake from the soil would require a storage volume of up to 10,000 L to support normal transpiration. There are some species of cacti, euphorbias and Mesembryanthemaceae that live transiently on water stored in cell vacuoles (e.g. Oppophyllum spp. or Prenia spp.), but their biomass production is then very low. Even the baobab tree (Adansonia digitata) is hardly able to use the water stored in its stem (Schulze et al. 1998b). The wooden structure of baobab and other “bottle trees” is rigid and cannot shrink sufficiently to maintain transpiration. The function of the thickened stems of baobab trees is to store carbohydrates and amino acid reserves in sufficiently hydrated cells.

During plant evolution, two strategies of water use have developed (Fig. 10.1a):

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig1_HTML.png — Fig. 10.1
Desiccation-tolerant and -intolerant plants. a Schematic presentation of the occurrence of desiccation-tolerant (poikilohydric) and desiccation-intolerant (homoiohydric) organisms. b Lichen growth dominated by *Teloschistes*, with almost no higher plants in the coastal region of the fog desert of Namibia. (Photo: E.-D. Schulze)

1.

Desiccation-tolerant, poikilohydric plants operate as a physical system of variable water content, which absorbs and loses water, depending on the humidity in the air (Chap. 6). When exposed to moisture from rain, dew or high humidity, these plants become fully active. They dry out with decreasing air humidity and become dormant during the time of desiccation. In this life cycle, there is an optimum water status. If the organism is too wet, diffusion of CO₂ from the atmosphere to the photosynthetically active cells is restricted and the rate of photosynthesis decreases with high water content (see Fig. 12.13). Thus, metabolism is restricted to the period when the tissue is wet, but not too wet, and diffusion of gases is possible. The most important representatives of this plant type of water use are algae, lichens and mosses (Fig. 10.1). Some mosses and lichens are able to obtain water from the ground by rhizoids and fungal hyphae via capillary forces but not via xylem vessels. Among flowering plants, there are also a few species that are desiccation tolerant—for example, Myrothamnus flabelliformis from the Namib Desert; Borya spp., native to Australia; and Craterostigma plantagineum in Africa, as described by Ziegler and Vieweg (1970) and Gaff (1971) (Chap. 6). These angiosperms, however, differ from non-vascular plants in that they cannot be activated by dew and high humidity as they have a cuticle, which restricts water uptake via the shoot (see also Burkhardt (2010)). These species are activated by water uptake through roots only.

2.

Desiccation-intolerant, homoiohydric plants are able to maintain a high and almost constant tissue water content that is independent of the conditions in the surrounding environment. There is a partitioning of labour between organs (Fig. 10.2): roots are specialised in uptake of water, the stem transports water and the green tissue (which may be leaves, phyllodes or phylloclades) assimilates CO₂ at the cost of evaporation. Homoiohydric plants have large vacuoles in their cells, which function, within a certain range, as short-term buffers for the cellular water status and thus stabilise the cell and plant water balance (Chap. 6). The leaf surface is covered with a cuticle composed of a lipophilic polymer, which is impermeable to CO₂ and H₂O but permeable to O₂. The leaf is connected to the free atmosphere via stomata, of which the aperture can be regulated. However, one developmental stage of intensive dehydration of cells also remains in these homoiohydric plants: the seed. In environments with high air humidity and low rainfall (e.g. the fog desert of the Namib, where there is no precipitation except for dew), higher plants are inferior—with respect to cover and growth—to lower plants (Fig. 10.1b).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig2_HTML.png — Fig. 10.2
Flow of water and assimilates in vascular plants. These are connected via the stem, which conducts the water and assimilate flows. In the stem, water and assimilates are transported by mass flow; in the leaf, there is a phase transition from liquid water to water vapour. At the same time, CO₂ is assimilated into soluble organic substances. Diffusion between the inside of the leaf and the atmosphere is controlled by the stomata. (Modified from Bonner and Galson (1952))

In this chapter, our focus is on homoiohydric plants, as they form the largest fraction of the terrestrial flora. While Chap. 6 presents the molecular basis for the responses of plants to water stress, this chapter will focus on the biophysical links between plants and the environment.

10.1.2 Availability of Water on Earth

The hydrological balance provides the overall conditions for plant growth on the Earth (Ward and Robinson 2000). If changes of water storage in the soil are disregarded, the hydrological balance consists of precipitation (P), evapotranspiration (E) and river discharge (F: river flow) (Eq. 10.1), which is fed by surface run-off and seepage (groundwater recharge).

(10.1)

The global distribution of these variables, as shown in Fig. 10.3, indicates tropical and temperate regions with a high surplus of rainwater (P > E + F) and arid regions with a rainwater deficit (P < E + F).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig3a_HTML.png — Fig. 10.3
Global distribution of components of the hydrological balance of the Earth. Global distribution of a precipitation (based on Schneider et al. (2014)), b actual evaporation (Miralles et al. 2011) and c groundwater recharge, equalling river flow (Döll 2009). d Climatic classification scheme, after Köppen and Geiger. (Kottek et al. 2006)

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig3b_HTML.png — Fig. 10.3
Global distribution of components of the hydrological balance of the Earth. Global distribution of a precipitation (based on Schneider et al. (2014)), b actual evaporation (Miralles et al. 2011) and c groundwater recharge, equalling river flow (Döll 2009). d Climatic classification scheme, after Köppen and Geiger. (Kottek et al. 2006)

Precipitation is the main input into the natural water cycle (neglecting the use of fossil groundwater for irrigation (Fig. 10.3a; Chap. 21)). It is determined by the position of the sun, the global circulation of air masses and the recirculation of local evapotranspiration. All of these processes result in high precipitation over the tropics, a minimum of precipitation in the subtropics and increased precipitation at temperate latitudes. Precipitation over the continents is also determined by the distance from the oceans (oceanity) and the size of the continents (continentality). The Gulf Stream, with its northern extension towards Europe (the North Atlantic Drift), also provides exceptionally favourable conditions of temperature and rainfall for the eastern part of the Americas, as well as for Europe. Precipitation decreases in the Arctic and Antarctic.

Surface evaporation (E) includes evaporation from surfaces and transpiration of vegetation. If the ground surface is covered by plants, free evaporation occurs only after precipitation, when the intercepted water (the amount of precipitation captured by the canopy and not reaching the ground) evaporates. In spring, before plant cover is achieved, arable fields lose water similarly to a wet surface until the topsoil layer dries off. The rate of evaporation decreases as the crop grows (Greenwood et al. 1992). Transpiration describes the amount of water lost from the plant by evaporation, and it is thus subject to physiological control, in addition to energy-driven evaporation (Chaps. 9 and 16). In addition, evaporation from the ground occurs in any stand, depending on the leaf area index (LAI) (Schulze et al. 1995). The sum of free evaporation and transpiration is called evapotranspiration (Fig. 10.3b). One distinguishes between potential evaporation, which is a function of the meteorological conditions, and actual evapotranspiration, which is additionally regulated by the plant cover. The global distribution of actual evapotranspiration shows a maximum in the tropical regions and roughly follows the distribution of precipitation. However, evapotranspiration is additionally influenced by the available solar radiation and mean wind speed (Chaps. 9 and 16). Thus, evapotranspiration decreases with decreasing available solar energy in the higher latitudes (north and south).

Surface run-off, water storage in the soil profile and river discharge of water close the hydrological balance (Fig. 10.3c). Evapotranspiration does not increase with precipitation without limits. Fig. 10.4 shows a saturation curve, with river discharge making up a larger proportion of the total precipitation as precipitation increases. Even at low precipitation, vegetation does not consume the total amount of precipitation. In arid climates, precipitation occurs as heavy rainstorms, with sometimes massive surface run-off. This water is thus not available for plants at the site of rainfall but may be stored at greater soil depth (e.g. in the Kalahari sands), serve plant growth downhill (e.g. in dry valleys) or reach the ocean via rivers.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig4_HTML.png — Fig. 10.4
Relations between river discharge, evapotranspiration and precipitation, based on river catchments. Evapotranspiration reaches a maximum that depends on the net radiation and surface conductance (for details, Chap. 16). Thus, the proportion of water that runs off or penetrates into the soil increases with increasing rainfall. (After Schulze and Heimann (1998))

10.1.3 Drivers of Water Flow Between the Soil and the Atmosphere

In soils and in plants, water is not freely available but is bound mainly to molecules or surfaces. In soil science and ecology, the following distinctions are made:

Constituent water comprises water that has been used in the plant metabolism.
Hydration water describes the layer of water molecules that is arranged as dipoles around ions. The strength of the binding increases with the charge of the hydrated ion and decreases with its radius. Thus, Na⁺ has a larger hydration layer than K⁺ (Lüttge and Higinbotham 1979). All polar groups of organic molecules have a hydration layer. Of the total water in a leaf, 5–10% is therefore not freely available.
Capillary water in cell walls and in the fine pores of soils is subject to capillary forces. The rise of the water column in a capillary (h, measured in metres) is, as a first approximation, inversely proportional to the radius (r) of the capillary, because the water mass and the gravitational force affecting it (πr ²hρg) must be balanced against the cohesive force produced by the surface tension (2πrσ _w cosα). Neglecting the contact angle, α (cosα = 1) for a 0° angle, the following equation applies (Nobel 2009):

$r\cdot h\cdot \rho \cdot g=2\cdot {\sigma}_{\mathrm{w}}$

(10.2)

where h is the height in metres to which the water column rises, ρ is the density of water (998 kg m⁻³ at 20 °C), g is the acceleration of the Earth (9.8 m s⁻²), r is the radius of the capillary (metres) and σ _w is the surface tension of water (0.0728 N m⁻¹ at 20 °C). Thus, the pressure (P) in a capillary, the so-called capillary force (as measured in Newtons) is:

$P=h\cdot \rho \cdot g=\frac{2\cdot {\sigma}_{\mathrm{w}}}{r}$

(10.3)

and the height of the meniscus is calculated as:

$h=\frac{2\cdot {\sigma}_{\mathrm{w}}}{\left(r\cdot \rho \cdot g\right)}=\frac{1.5\times {10}^{-5}{\mathrm{m}}^2}{r}$

(10.4)

In a clean (lipid-free) glass capillary 3 μm in diameter (with a 1.5 μm radius), water rises 10 m. In a xylem vessel of 30 μm (typical for tracheids of conifers), water rises by capillary forces only 1 m, and in the 300 μm trachea of deciduous trees the water rises only 0.1 m via capillarity. Thus, the capillary force is not sufficient to lift water into the canopy of trees and therefore does not provide the lifting power required for the flow of water.

Cell water is osmotically bound (e.g. in the plant vacuole). The osmotic pressure, П, depends on the number of particles per mole (n), the concentration (c _n), the gas constant, R (8.3144 Pa m⁻³ mol⁻¹ K⁻¹) and the temperature, T (in Kelvins):

$\varPi =n\cdot {c}_n\cdot T\cdot R$

(10.5)

In contrast to Chap. 6, the osmotic pressure is expressed here as positive pressure measured in Megapascals. This concept is based on an experiment that also demonstrates the phenomenon of osmosis: A closed chamber, called an osmometer (Pfeffer 1877), is divided by a semipermeable membrane into two compartments. The membrane allows passage of water molecules but is impermeable for ions. One compartment of the chamber is filled with distilled water, the other with a salt solution. In this case, free water is able to flow into the chamber with the salt solution, building up hydrostatic pressure. The level of water rises in the salty compartment, compared with the chamber containing free water, until the water column balances gravity. The height of the water column corresponds to the osmotic pressure. It is 2.48 MPa for a 1 molar solution.

To describe the flow of water between the very heterogeneous compartments of the plant and the environment, it is necessary to quantify the availability of water uniformly. This is possible by the definition of a common force for water transport, the water potential (Chap. 6):

$\varPsi =\frac{\left({\upmu}_{\mathrm{w}}-{\upmu}_{\mathrm{o}}\right)}{V_{\mathrm{w}}}=\left(\frac{V_{\mathrm{w}}}{V_{\mathrm{w}}^{\mathrm{o}}}\right)\cdot \varPi$

(10.6)

where μ _w is the chemical potential in the system (J mol⁻¹) and μ _o is the chemical potential of a reference system—that is, of pure liquid water at a given temperature and at normal pressure (atmospheric pressure). Dividing the difference of the chemical potentials (μ _w – μ _o) by the molar volume of liquid water (V _w) results in the water potential being defined in units of pressure (in Pascals) (Slatyer 1967; Walter and Kreeb 1970). Thus, the water potential describes the driving force for water movement in units that can be measured easily. V _w is slightly dependent on temperature and atmospheric pressure. ${V}_{\mathrm{w}}^{\mathrm{o}}$ expresses the molar volume of pure water (Π = 0) in physical normal conditions.

In the gaseous phase the water potential is proportional to the relative humidity (Nobel 2009):

$\varPsi =\frac{T\cdot R\cdot \ln \left(\frac{e}{e_0}\right)}{V_{\mathrm{w}}}$

(10.7)

where e/e ₀ expresses the vapour pressure of bound water (e.g. in solution or solid material) relative to that of free water. e/e ₀ corresponds to the relative humidity. The proportion e/e ₀ is also called water activity—describing, for example, the degree of swelling of colloids—and thus characterises the conditions for life of microorganisms or poikilohydric plants (Walter and Kreeb 1970).

As the chemical potential of bound water, μ _w, is more negative than that of free water, μ _o (energy has to be added to change, for example, bound water in a salt solution into the state of free water), the water potential has a negative sign. Water movement occurs from sites with high (more positive) potential to sites with low (more negative) potential. Thus, the water potential describes the state of water of the atmosphere, soils, plants or particles, and the water potential difference between compartments is the driving force for water flow.

Using the water potential, it is possible to describe water status and water flows in single-phase systems (e.g. in a plant), as well as in phase transitions (e.g. water uptake from soils; Chap. 6). In a cell with good water supply and without any water flow, the osmotic pressure is compensated by the counter-pressure of the cell wall—the turgor pressure (P _c)—which can be measured as a positive pressure in a cell (except for xylem cells where it is negative because of their special structure). With freely available water, the water potential of the cell or tissue is zero. With decreasing water content of the cell, the turgor pressure decreases and the osmotic pressure increases because of the increasing concentration of the residual solution in the cell. The difference, P – П, corresponds to the water potential, Ψ, which becomes increasingly more negative when the cell dries out. In a tissue under such conditions, water flows, for example, from the cell wall into this cell until the water potential gradient equilibrates. If desiccation of the cell continues, the water potential may become equal to the osmotic pressure (Π = −Ψ). At this point of turgor loss, plants start to wilt and cells start plasmolysis.

The water potential of the living cell is described as:

$\varPsi ={P}_{\mathrm{c}}-\varPi -\tau +\rho \cdot g\cdot h$

(10.8)

where τ represents an additional force describing the binding of water in the membrane-free matrix of the cell wall and their coating with water molecules (the so-called matrix potential; Sect. 10.2.1), which depends on surface forces and not on the number of particles in the solution, as in the case of П. This is important under saline conditions (Kramer and Boyer 1995). ρ g h is the water pressure, with ρ describing the density of water, g the gravitational force (9.807 m s⁻² at 45° latitude) and h the height of the meniscus above-ground (in metres; see Eq. 10.3). Thus, the water potential is dependent on the turgor pressure, the osmotic pressure, the chemical binding of water, surface properties and gravity, which is particularly important for tall trees Eq. 10.8, where the water potential is different in the root and shoot, depending on the height. Many authors also use the expression:

${\varPsi}_{\mathrm{W}}={\varPsi}_{\mathrm{P}}+{\varPsi}_{\varPi }+{\varPsi}_{\mathrm{g}}$

(10.9)

where ${\varPsi}_W$ is the water potential in the cell, images/72100_2_En_10_Chapter/72100_2_En_10_IEq5_HTML.gif

is the turgor, ${\Psi}_{\mathtt{\varPi}}$ is the osmotic potential and images/72100_2_En_10_Chapter/72100_2_En_10_IEq7_HTML.gif

is the potential due to gravity (Chap. 6, Sect. 6.2).

If the water potential between the plant, soil and air is balanced, there is no net transpiration flux. In nature, this may occur at night or during early morning at high air humidity. Therefore, the early morning predawn water potential (Ψ_predawn) in a plant is used to characterise the water conditions of the soil in the zone from which the roots obtain their water, which is in equilibrium with the leaf under conditions of no transpiration. With transpiration, a gradient in water potentials develops between the soil and the atmosphere. The midday water potential (Ψ_midday) thus decreases far below the predawn level and may reach about 10 MPa.

In the continuum of water potentials between the soil and the atmosphere, water follows the water potential gradient (from high to lower potential, i.e. to increasingly more negative values), whereby the flow rate is limited by flow resistances depending on the characteristics of the soil and the types of tissue. In addition, there is a phase transition in the leaf from the liquid to the vapour phase at the site of evaporation in the leaf, with the rate of diffusion in the vapour phase being determined by the water vapour pressure of the atmosphere (e/e ₀). In the soil–plant–atmosphere continuum (SPAC), the highest water potential gradient is between the cell walls of the leaf mesophyll where water evaporates and the atmospheric air, because of the extremely low water potential in the air (see Eq. 10.7 and Table 10.1). The physical description of leaf water status does not consider that the cellular water status might be additionally regulated by membrane-localised pores or valves—the aquaporins (Chap. 6)—which facilitate water transport across lipophilic membranes.

Table 10.1

Relationship between water potential and relative humidity. Following Eq. 10.7, air humidity can be measured in a closed vessel in which air equilibrates with salt solutions of different concentrations. Such an experiment can also be used to study particular processes—for example, moulds can establish at relative air humidity of >70%. (Walter and Kreeb 1970)

Relative humidity (%)	Water potential (MPa)	Condition in soil or plant
100	0	Field capacity of soil
99	−1.35	Matrix potential at permanent wilting point
98	−2.72	Strong water stress in a plant
90	−14.1	Lowest measured water potential in a desert plant
80	−30.1	Initiation of photosynthesis in lichens
70	−48.1	Initiation of respiration in moulds
50	−93.3	Ambient air in an office

In a closed system, the water potential is related to the relative humidity of the surrounding air (see Eqs. 10.7 and 10.17). Thus, it is interesting to compare the relative air humidity with the water potential under equilibrium conditions, and various plant responses (Table 10.1). Most plants reach a permanent wilting point (Fig. 10.6) already at −1.5 MPa soil water potential when the relative humidity decreases to 99% inside the mesophyll. Photosynthesis is initiated in lichens at 80% relative humidity, and respiration in fruit moulds is activated at 70% relative humidity. Thus, the atmosphere is too dry for active live under average conditions.

A cell in a plant tissue is embedded in a water potential gradient from the soil to the atmosphere as created by the water flow in the xylem, along a flow resistance (Fig. 10.5). The cell water potential equilibrates with the water potential in the xylem. Thus, the turgor pressure changes with the xylem water potential, and the cell may respond to these pressure changes by adjustment of the osmotic pressure in the vacuole. The water flow in the soil–plant–atmosphere continuum will be further discussed in Sect. 10.2.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig5_HTML.png — Fig. 10.5
Water status of a cell in a plant tissue

10.2 Water Transport from the Soil to the Plant

10.2.1 Water Uptake

Precipitation seeps through the soil profile and may reach groundwater. The amount and rate of water movement in the soil depends on the soil type, the pore size and the water saturation. The field capacity describes the content of water, which is retained against gravity—that is, it is the amount of water that does not drip out of a flower pot after watering (field capacity is lower for sand than for loam). However, this water is only partially available for plants, because a proportion is bound as hydration water or by capillary forces—the matrix potential (Ψ_m) of the soil (see Eq. 10.4). In pores of 5 nm (e.g. pores in secondary cell walls of higher plants), the matrix potential reaches 3.0 MPa. It decreases in pores with a radius of 500 μm to only 30 Pa (Schachtschabel et al. 1998). Threshold values have been defined for the characterisation of soils on the basis of the relation between the water potential and the soil water content (Fig. 10.6a) (Or et al. 2012).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig6_HTML.png — Fig. 10.6
Changes in water status and plant-available water in sandy and loamy soils. a Depending on the texture, the water potential at field capacity is −0.05 MPa (in sand) and −0.015 MPa (in loam). Convention sets the permanent wilting point (PWP) of agricultural systems at −1.5 MPa. Plants that live in wet or dry conditions may have wilting points at −0.7 MPa and −3 MPa, respectively, but the additional available water resulting from a shift in the wilting point is small. The limit of hygroscopically bound water is set at −5 MPa. The difference between field capacity and hygroscopically bound water is the exchangeable water, only a proportion of which is available for the plant (from Larcher (2003)). b Change in the amount of water available to a plant, depending on the soil texture. In arid areas, sandy soils contain more water that is available to a plant than in a loamy soil, as the water is not as strongly held by capillarity in sand as it is in loam. In contrast, in areas of high rainfall, the amount of water that can be stored by a sandy soil is less than that in a loamy soil (Modified from Walter (1960)). c Distribution of vegetation in the Namib Desert (Asab, Namibia) with *Acacia karroo*, *Aristida sabulicola* and *Acacia ciliata* growing on the sand dunes (with high water availability and low salt) and vegetation-free areas on the plain, where loam and clay soils have low water availability and higher salt concentrations. (Photo: E.-D. Schulze)

Loam and clay soils differ not only in the amount of available water (for loam it is almost ten times higher than for sandy soils), but also in the amount of water that is not available for plants in dry soils. This has consequences for the plant availability of water with decreasing precipitation (Fig. 10.6b). In areas with high precipitation, sandy soils are drier habitats than loamy or clay soils because sandy soils retain less water, owing to the larger pore sizes. At low precipitation the limit at which water can no longer be taken up by the plant is reached earlier in clay because the capillary and matrix forces are reached sooner than in sandy soils. In arid regions, sandy soils are therefore “moister” habitats for plants than clay soils. This is modified in nature by the frequency of precipitation (Fig. 10.6c).

Water transport from the soil to the root is determined not only by the water potential gradient but also by the hydraulic conductivity, which depends on the soil water content and soil texture. Roots may grow towards the water or “wait” until water flows from the soil to the root. In most plants, water transport to the roots is additionally facilitated by fungal hyphae forming the so-called mycorrhizae (Marjanovic and Nehls 2008) (Chap. 19), in which water is mainly transported towards the root by capillary forces. The availability of water in the soil determines the surface area of roots needed to provide leaves with a certain amount of water.

Water transport in soils is different in saturated soils (water content above field capacity) and unsaturated soils (water content below field capacity; water moves as vapour). In saturated soils, water flow follows Darcy’s law (Darcy 1856). Darcy studied the water pressure in the wells of the city of Dijon, France. In this case the flux, or Darcy velocity (v, in volume per time and area, measured in cubic metres per hour per square metre or in metres per second), is proportional to the hydraulic gradient (dh/dx: change in height per change in the length of the flow path, which corresponds to the potential gradient) and the saturated soil water conductivity (k _s), which depends on soil texture, where soil texture describes the grain size of soil particles (stone, sand, silt, clay):

$v=-{k}_{\mathrm{s}}\left(\frac{\mathrm{d}h}{\mathrm{d}x}\right)$

(10.10)

In stony soils k _s is >10⁻³ m s⁻¹, in sand it is >10⁻⁵ m s⁻¹, in silt it is 10⁻⁷ m s⁻¹ and in clay it is 10⁻⁹ m s⁻¹.

In unsaturated soils, the rate of flux is much lower than that in saturated soils. It depends on the unsaturated soil water conductivity, k _Θ, which decreases with the soil water content, Θ, and is determined by the potential gradient, ΔΨ, over the distance, x.

$v={k}_{\varTheta}\cdot \kern0.5em \left(\frac{\varDelta \varPsi}{\varDelta x}\right)$

(10.11)

This equation is analogous to Eq. 10.10, but the water potential is the driving force, which in turn is related to the relative humidity in soil pores (Eq. 10.7). The values for k _Θ in silt soils range between 10⁻¹³ near saturation and 10⁻¹⁷ cm s⁻¹ close to the wilting point. Soils are not homogeneous but structured in horizons and differentiated within the horizons in more or less dense aggregates. Thus, the hydraulic conductivity of soils varies over a very short distance with texture, aggregation and soil water content (Horn 1994). Thus, even within a single soil profile, water availability is highly variable, which results in high variability in the root architecture of plants. In dicots, the seminal root is long lived and, as a tap root, explores deeper soil horizons with an extensive root system. In horizons where nutrients and water are available, a dense adventitious root system is formed. In contrast, in grasses the seminal root is short lived. A dense adventitious root system develops near the soil surface (Fig. 10.7). There is an interaction between water and nutrient availability on the one hand and the hydraulic structure on the other (Ewers et al. 2000).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig7_HTML.png — Fig. 10.7
Root architecture of dicotyledonous and monocotyledonous plants. Root distribution of a the dicotyledonous *Oxytropis campestris* and b the grass *Lolium perenne*. (redrawn from Kutschera and Lichtenegger (1992))

The soil layer from which plants gain their transpiration water may be determined with stable isotopes of hydrogen, D = deuterium, and oxygen, ¹⁸O. The stable isotope values for hydrogen and oxygen (δD and δ¹⁸O) increase with the temperature of precipitation (Dawson 1993)—that is, winter rains have a lower stable isotope value than summer rains. In addition, isotopes enrich at the soil surface, as water molecules containing heavier isotopes evaporate more slowly than molecules with lighter isotopes. Measuring the isotope ratios in (D/H or ¹⁸O/¹⁶O) xylem water and in water along a soil profile provides information on the horizons from which species may take up their water.

Establishing the origin of transpired water (water uptake) with reference to the soil depth is most clear in regions with seasonal rainfall at different temperatures. An example is given in Fig. 10.8a for the Colorado Desert in the south-western USA (Ehleringer 1993, 1995). Summer and winter rains show very different δD values, and groundwater is even more depleted in deuterium than winter rain because of isotope fractionation during seepage of water to a greater depth. Various plant species use water sources from different depths. In this subtropical “warm” desert (Fig. 10.8b), annuals and succulents use summer rains, which reach the Colorado Desert as sporadic subtropical fronts (note: in Mediterranean winter-rain regions, annuals use mainly winter rains). In contrast to the summer annuals, deeper-rooting perennials (usually, evergreen herbaceous plants and shrubs) utilise the water of winter rains or groundwater. In between these two contrasting types there is a group of moderately deep-rooting perennial plants, which use the water as it percolates through the soil profile. At times between these periods of precipitation, there are longer dry periods when these plants may even shed their leaves (i.e. they are deciduous). The differentiation of species according to their source of water applies not only to the Colorado Desert but also to other dry regions (e.g. the temperate semi-deserts in Argentina; Schulze et al. 1996b). Under arid conditions, water may also rise in the soil by capillary forces, but this will occur only above a water table. The hydraulic redistribution by plants is explained below (Fig. 10.11).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig8_HTML.png — Fig. 10.8
Stable isotope values of hydrogen for plant types and groundwater in the Sonoran Desert. a Hydrogen isotope values in water from the xylem of different types of plants growing in the Sonoran Desert (*blue circles*), compared with the hydrogen isotope values in summer and winter rainfall (*blue bands*), as well as in groundwater (*black line*) (after Ehleringer (1993)). The δD value is calculated from the D/H ratio of the sample in comparison with a standard [(D/H)_sample/(D/H)_standard – 1], where water from deep oceans is used as the standard (V-SMOW = Vienna standard mean ocean water). In the case of the Sonoran Desert, annuals use the summer rainwater, which has a high δD value depending on the temperature, almost exclusively. In contrast, deep-rooted perennials almost exclusively use water from winter rainfall, which has a low δD value due to the lower temperatures. The δD value of water from the xylem of plants shows from which soil level they obtain the water. b Vegetation in the Sonoran Desert close to Oatmans, South Nevada, with summer annual plants *Pectis papposa* (Asteridae, C₄ plants); perennial woody plants, *Ambrosia dumosa* (Asteridae, C₃ plants); and perennial deep-rooted *Larrea tridentata* (Zygophyllaceae). (Photo: E.-D. Schulze)

Water uptake by plants from the soil occurs in a zone behind the apex of the root tip where root hairs develop and the root cortex is not yet suberised (Steudle 1994, Steudle and Peterson 1998). In addition, water is taken up in meristematic regions of lateral roots. In older parts, the root is differentiated (Fig. 10.9a) into an epidermis below which a suberised layer called the exodermis exists in many species, followed by the root cortex, a heavily suberised or lignified endodermis (the Casparian band) and the central cylinder (the stele) with the xylem vessels and the phloem. Water follows (as a first approximation) the water potential gradients from the soil to the xylem, but along various pathways (Fig. 10.9b). In the region of the root cortex, water may (a) flow in the cell wall (apoplast), (b) move from cell to cell via the plasmodesmata (symplast) or (c) move across the cells (via the transcellular path). At the endodermis (and probably also at the exodermis), water must be moved through the symplast of specialised unsuberised transmission cells. In the undifferentiated root tip, where the endodermis does not yet exist, water may enter via the apoplast and the symplast of growing cells. Thus, water transport in the root may be described by a model with a series of parallel resistances connected by serial resistors. The hydraulic conductivity in each cell layer (Lp_z, measured in metres per second per Megapascal; Eq. 10.12) may be described by:

${\mathrm{Lp}}_{\mathrm{z}}={\gamma}_{\mathrm{c}}\cdot \kern0.5em \frac{{\mathrm{Lp}}_{\mathrm{c}}}{2}+{\gamma}_{\mathrm{c}\mathrm{w}}\cdot \kern0.5em \frac{{\mathrm{Lp}}_{\mathrm{c}\mathrm{w}}}{\varDelta x}\kern0.5em \cdot \kern0.5em \frac{\varDelta x}{d}$

(10.12)

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig9_HTML.png — Fig. 10.9
Water flow through the root. a Cross-section through a maize root in which the lignin and lipids in the exodermis and endodermis are stained with berberin sulphate. b Schematic cross-section of a root showing routes of water and nutrient transport. The suberised Casparian bands appear as *black dots* in this cross-section, showing their position in the cell wall. *Blue arrows* mark clearly different paths that water can take. (After Steudle (1994))

where Lp_c describes the hydraulic conductivity of cellular membranes and the factor 2 considers the fact that two membranes per cell must be crossed. Lp_cw is the hydraulic conductivity of the cell wall, γ _c and γ _cw are the amounts of cytosol and cell wall at the cross-section in the direction of the flux (γ _cw + γ _c = 1), Δx is the width of a cell layer and d is the thickness of the tissue. Equation 10.12 shows that the relevant fluxes may be distributed differently according to the structure of the root. The distribution of the pore size in the cell wall and the size of the hydrated ions determine the conductivity. Pores in the inter-micellar space of the cell wall are about 5 nm; the space between the cellulose fibrils is about 10 nm. In comparison, a water molecule diameter is about 0.3 nm, Na ions with their hydrated shell reach 0.5–0.7 nm, K ions measure 0.4–0.5 nm and a glucose molecule measures about 0.75 nm (Lüttge and Higinbotham 1979). Even though the cross-section available for apoplastic transport is much smaller than that for the cytosol, it depends on the conditions if the flux is hydraulic (following the water potential gradient of transpiration) or metabolically regulated following an osmotic gradient. Thus, the root is not a uniform structure (Steudle and Peterson 1998).

If the apoplast path is interrupted (e.g. by strong suberisation of roots), the cellular component dominates (Michael et al. 1997). The flow of water through the cell membrane in the cellular transport path is affected by aquaporins (proteins that act as valves in the hydrophobic membrane of the cell) (Tyerman et al. 1999) (Chap. 6).

Even though water transport through the endodermis is metabolically regulated, water flow can be inhibited by only about 50% by mercury, which binds to sulphur-containing amino acids in the aquaporins. Also, antisense-plants lacking aquaporins maintain 40% of the hydraulic conductivity of control plants (North and Peterson 2005).

The water transport across the Casparian band results in a positive pressure, which is visible on cut stems as emerging droplets. The root pressure is caused by an osmotic gradient of about 100 mM of inorganic ions, typically under conditions with high moisture or low/no transpiration. This results in a pressure in the order of 1–2.5 MPa and may lift water from the root to the leaves up to a height of 25 m (Nobel 2009; Lambers et al. 1998). However, for water transport in dry air, this process is not sufficient to carry enough water; it may however be important in healing cavitation of xylem elements (Fig. 10.15).

The differentiation of root anatomy is, from an ecological point of view, a response to the conditions of water flow between the soil and root, and the uptake of nutrients. Water uptake normally does not limit the supply of water to the plant in moist soils. The water potential in the shoot is, of course, also determined by transpiration. Thus, in moist soils, the water potential in the xylem decreases (becomes more negative) with the amount of water that is transported through the system (Fig. 10.10, curve A), the slope representing the hydrologic resistance (gradient/flux) or hydraulic conductance (flux/gradient) of the root and the stem. In ecology, “conductance” is the preferred variable because it is proportional to the flux (Cowan 1977). In dry soils, corresponding to the low flux and the very low conductivity in the unsaturated soil, a dry zone may develop around the surface of the root—that is, further supply from the soil may, in this case, be the limiting factor (Michael et al. 1999). This state would be visible in the leaf by a strong reduction of water potential in the xylem without an associated increase in the flux through the vessels (Fig. 10.10, curve B). A corresponding turgor loss is expected to occur in the root tip under such conditions, leading either to osmotic adjustment or to production of the stress hormone abscisic acid (ABA) (Chap. 6). Conversely, there are situations where changes in water transport may occur without changes in water potential (Fig. 10.10, curve C), —for example, by water availability in specific horizons. Here, the water potential gradient between the soil and xylem reaches a magnitude that allows a flux via additional surfaces (i.e. other roots or root regions) that were not participating in water uptake because the potential gradient was too low. Thus, the hydraulic conductance of the root, and thus water uptake, is variable, and it is plant regulated through the root architecture (Ewers et al. 2000) and the molecular responses to drought in the root tip.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig10_HTML.png — Fig. 10.10
Schematic diagram of changes in xylem water potential with increasing water flow. A The hydraulic conductance is constant. B The hydraulic conductance falls when the interface between root and soil dries out. C Additional roots contribute to the transport as the soil dries

The site for water uptake also harbours a potential leak. The plant cannot seal itself against the soil. “Unprotected” regions are the root tips and the axial meristems of lateral roots (i.e. the regions of water uptake into the roots). Also, the transmission cells in the endodermis and in the exodermis may transport water in both directions—that is, roots not only are able to take up water from the soil but also may lose water to the soil. This is ecologically important at low transpiration. During the hydraulic lift (also called hydraulic redistribution), water is taken up from wet soil (often in deep horizons) and moved by a water potential gradient to another (mainly the upper) soil horizon, where it is released into the dry soil (see reviews by Neumann and Cardon (2012) and Prieto et al. (2012)). Water release of plants to the soil was first observed in dry climates (Richards and Caldwell 1987), but it is also important in temperate climates. For Acer saccharum it was observed (Dawson 1993) that the isotopic composition of soil water in the region of the canopy corresponds not to the rainwater but to the much deeper groundwater (Fig. 10.11).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig11_HTML.png — Fig. 10.11
Stable isotope values of hydrogen (δD values) as a marker of the source of water. The δD value is calculated from the D/H ratios of the sample in comparison with a standard [(D/H)_sample/(D/H)_standard – 1)], where water from deep oceans is used as the standard (V- SMOW = Vienna standard mean ocean water). The δD value in vegetation is between the high value in rainfall and a significantly lower δD value in the groundwater. The lower δD value in the xylem water shows that *Acer saccharum* derives its water from the groundwater. The soil water (0–30 cm deep) shows a gradient in the δD value, from the low value near the trunk to a high δD value away from the trunk. The water with the lowest δD value can only have come from groundwater that is transported by the roots and is released into the soil, during the so-called hydraulic lift. The vegetation reacts differently to the water availability depending on how deeply the roots penetrate. While *Fragaria virginiana* is able to use the “lifted” water, the roots of *Holcus lanatus* do not go that deep. (After Dawson (1993))

The smaller the distance to the root of the tree is, the more similar the isotopic composition of the xylem water of the herbaceous vegetation is to that of the tree xylem water. Obviously, during the night, larger amounts of water are transported by tree roots from the moist subsoil above the water table into the dryer topsoil. The isotopic signature of the water of the topsoil changes correspondingly. The water is utilised during the day not only by the tree but also by the vegetation covering the ground in the shade of this tree (“water parasitism”). In this example, 30–60% of the water in the xylem of this ground flora originates from the hydraulic lift of the tree roots. This example may explain the often luxurious vegetation of herbaceous species in the shadow of trees in semi-arid regions.

The reverse process, the inverse hydraulic lift (Schulze et al. 1998a; Burgess et al. 2001), is ecologically just as important as the hydraulic lift. In arid regions the lower soil layers may very rarely be moistened as precipitation is sufficient only to wet the upper soil layers to field capacity. The “wave of water” produced in the soil by precipitation penetrates only a few decimetres (in silt) or metres (in sand). Lower soil layers remain permanently dry unless there is groundwater. Nevertheless, roots are able to penetrate such dry soil to a considerable depth by the inverse hydraulic lift (Fig. 10.12; Canadell et al. 1996).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig12_HTML.png — Fig. 10.12
Depth of roots in global vegetation types. a Maximum observed rooting depth of different types of vegetation. Each line represents a single measurement of a particular plant species. The numbers show the maximum depths that are beyond the y scale (from Canadell et al. (1996)). The deepest known root depth was measured in the Kalahari Desert from *Acacia erioloba* when bore holes were sunk to the water table. b *Acacia erioloba* savanna with perennial C₄ grasses *Aristida* and *Stipagrostis* (see Schulze et al. (1996a)) in the Kalahari Desert, north of Uppington, South Africa. (Photo: E.-D. Schulze)

The maximum rooting depth in deserts and savannas is more than 50 m. The absolute record for observed root depth is 68 m in the Kalahari, where the groundwater is more than 100 m deep, covered by dry sand, and it is expected that roots are able to penetrate to that depth. Roots up to 100 m deep have not yet been found. The deepest roots were found by chance during construction of a well. Penetration of roots to such a depth in dry soil is possible only by transport of water from the moist topsoil (i.e. the inverse hydraulic lift)—that is, the root must be kept wet in a very dry rhizosphere. Without additional transport of water, the root tip, which is protected only by mucilage, would desiccate in the dry soil.

Even though water uptake occurs predominantly in roots, liquid water may be taken up by shoots even from fog and dew via lenticels of the bark (Klemm 1989) and via water films of the stomata (Burkhardt 2010).

10.2.2 Xylem Water Transport

A stem structure distinction is made between herbaceous and woody species, but many transitions exist. In fact, the xylem elements of all herbaceous species contain lignin to achieve the needed rigidity of vessels, which operate under tension. Thus, Schweingruber and Büntgen (2013) concluded that the classification between woody and herbaceous species is not supported by wood anatomy. One may also distinguish between stems according to the structure of the vascular systems. In closed vascular bundles, the initial meristem between xylem and phloem cells, the cambium, terminates cell division. The xylem and phloem are surrounded by a vascular bundle, the bundle sheath, most conspicuously developed in grasses (Fig. 10.9). In contrast, open vascular bundles exist in most dicots where the cambium remains active and produces xylem cells towards the plant interior and phloem cells towards the outer periphery of the stem. An example is the herbaceous species Arabidopsis thaliana, which contains a stem anatomy identical to that of woody stems (Fig. 10.13a). Other herbaceous species (e.g. Polygala alpestris) even exhibit seasonal growth rings (Schweingruber et al. 2013). In trees and shrubs, a distinction is made according to the arrangement and size of vessels during the course of the growing season. In diffuse porous wood (Fig. 10.13b), vessels and tracheids of different diameter are formed according to the demand of water flow at any time of the season (e.g. in Betula pendula). The wood is very similar to that of Arabidopsis thaliana, except that large vessels exist only in trees and shrubs. In ring-porous wood, a ring of very large vessels is formed when growth is initiated after winter and new leaves develop (Quercus robur; Fig. 10.13c). With ongoing seasons, smaller vessels and even tracheids follow.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig13_HTML.jpg — Fig. 10.13
Wood anatomy of plant types. Cross-sections of a the herbaceous stem of *Arabidopsis thaliana* with secondary xylem cells, b the diffuse porous wood stem of *Betula pendula* and c the ring-porous *Quercus robur*. d Coniferous wood of *Abies alba*. Ca cambium, Co cortex, Ew early wood, Fi fibre, Lw late wood, *Lwv* late wood vessel, Pa parenchyma, Pe phellogen, *Ped* periderm, Ph phloem, Ra ray, Sc sclerid cell, Se sieve elements, Tr tree ring, *Tra* tracheids, *Wwv* early wood vessel, Xe xylem element, Xv xylem vessel. (Anatomical sections by F. Schweingruber)

In all cases the xylem elements are formed by an open meristem where the cells die after elongation. The cambial activity is needed in long-lived species because phloem elements are relatively short lived. Since cell division of the cambium results in phloem and xylem elements, the dead xylem cells accumulate and remain functional for water transport, connecting the living root central cylinder with the living leaf mesophyll. The size of the vessels is determined by a plant-hormonal balance in the cambial layer (see Schweingruber et al. (2013)). Since there is also a balance between the leaf area index (LAI) of transpiring leaves and the total xylem area that transports water, the long-lived xylem elements become dysfunctional if they do not participate any more in water transport. Under these conditions, the remaining meristematic cells of the wood seal the xylem elements mainly with tannins to make these elements resistant against fungal attack. Thus, we distinguish between an outer ring of xylem elements (the sapwood), which participates in the water flow, and an older inner part of the wood (the heartwood), which is not conducting water but stabilises the stem (Fig. 10.14). Because of the tannins, this wood is generally darker-coloured, but there are species where the heartwood is not clearly visible (e.g. Picea abies). It is the heartwood that gives a species the physical strength for larger structures. With heartwood formation, most meristematic cells also die, but some meristematic cells may remain alive for more than 100 years (in Carpinus betulus) (Fritzsche 1910). It should be noted that young trees contain only sapwood. Heartwood formation occurs when the stem area increases beyond a required sapwood area. Thus, the number of conducting elements in woody plants is regulated by the annual increase of new elements as well as by the transition of old elements from sapwood to heartwood (Fukuda 1997). Only the sapwood area conducts water.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig14_HTML.png — Fig. 10.14
Wood anatomy. a Section of a piece of wood with its main structures. b Cross-section of *Robinia pseudacacia*, with light-coloured sapwood and dark-coloured heartwood. (Photo: E.-D. Schulze)

The cambial activity is not constant but highly dependent on the growing conditions. Cambial activity ceases only during winter and during dry periods. This change in cambial activity results in growth rings (called tree rings in shrubs and trees) of variable activity. Since the anatomy of wood does not change any more after cambial division and elongation, tree rings are a “fossil” record of the growing conditions in the past and have been used to reconstruct the climate, on the basis of statistical models, explaining present-day ring width with climate conditions during the growing season (Vaganov et al. 2006). There are species that do not form a ring structure but terminate growth under conditions of drought without an obvious anatomical signal. Trees in humid tropical regions without any dry season also have no annual tree ring structure but exhibit visible changes in growth activity.

Xylem water transport follows the water potential gradient between the root and leaf during the day. The hydraulic conductivity of the xylem is relatively high. The question of the physical conditions in a capillary with negative pressure of more than 10 MPa has been a topic of research for many years. Böhm (1893) was probably the first to postulate that the cohesion between water molecules is sufficient to achieve a continuous water column in the xylem vessels under tension (cohesion theory). With the measurement of negative pressures of more than 1 MPa in xylem vessels (Wei et al. 1999) and the observation that the tension changes with the flux through the xylem vessels, the cohesion theory has also been confirmed by measurements.

Biophysically the water flux in a xylem vessel, J _x (in cubic metres per second), is described by the Hagen–Poiseuille law for laminar flows and depends on the radius, r, of the xylem vessel, the viscosity of the liquid (η = 10⁻³ Pa s for water) and the hydrostatic gradient, dP/dx (in Pascals per metre):

${J}_{\mathrm{x}}=\left(\ \frac{\ \pi {r}^4\ }{8\eta }\ \frac{\mathrm{d}P}{\mathrm{d}x}\right)$

(10.13)

The flow is in the direction of the decreasing hydrostatic or water potential gradient (from less negative to more negative values). The flow must be sufficiently slow so that the conditions of laminar flow (in contrast to turbulent flow) are maintained, to avoid rupture of the water columns. The potential gradient required to transport a certain volume flow (e.g. 1 mm s⁻¹) across the cell wall is very high, about 3 × 10⁵ MPa m⁻¹, according to Eq. 10.13 (Nobel 2009). Thus, major forces are required to move water through the cell wall of the leaf mesophyll, which leads to relatively slow movement of water in the xylem.

In addition to the regulation via the vessel diameter, the volume flux per time and unit area, I _x (measured in metres per second), in the xylem under a pressure gradient (ΔP/Δx) is determined by the area of the cross-section per vessel and the number of xylem vessels, n, per organ:

${I}_{\mathrm{x}}=\frac{r^2}{8\eta}\kern0.5em \cdot \kern0.5em \frac{\varDelta P}{\varDelta x}\kern0.5em \cdot \kern0.5em n={\mathrm{Lp}}_{\mathrm{x}}\cdot \varDelta P$

(10.14)

where I _x is measured in (square metres × Pascals)/(seconds × Pascals × metres) = (metre per second) and Lp_x corresponds to the hydraulic conductivity in the xylem (measured in metres per second per Pascal). The axial hydraulic conductivity is related to a 1 m length of xylem and thus has different dimensions from the Lp of the membrane.

Following Eqs. 10.13 and 10.14, plants have many possibilities to regulate the flux in the xylem and thus the water potential gradient, or the water potential gradient and the concomitant flux (Gartner 1995).

The vessel radius varies between 500 μm in lianas to approximately 100 μm in ring-porous woody plants (e.g. oak) and 10–40 μm in the tracheids of conifers (Table 10.2); a larger radius allows a considerably higher volume flux. In tropical lianas, in ring-porous woody plants and in dicotyledonous herbaceous plants, water taken up by the root reaches the transpiring leaf in less than 1 h. In contrast, it takes 2–3 months for the water taken up by the roots to reach the tip of a 100 m high Sequoia gigantea, because of the low average rate of flux during only part of the day.

Table 10.2

Anatomy, conductance and water flow in the xylem. Vessel radius and length, and maximum capillarity of vessels from different types of plants according to Zimmermann (1983), Carlquist (1991) and Nobel (2009)

Plant type	Specific conductance for water (m s⁻¹ MPa⁻¹)	Maximum rate (m h⁻¹)	Vessel diameter (μm)	Vessel length (m)	Capillarity (kPa)
Conifers	5–10 × 10⁻⁴	1–2	10–40	0.002–0.005	29–7
Diffuse porous angiosperms	5–50 × 10⁻⁴	1–6	5–150	1–2	58–1.0
Ring-porous angiosperms	50–300 × 10⁻⁴	4–44	10–600	10	29–0.5
Herbaceous plants	30–60 × 10⁻⁴	10–60	10–500	1–2	29–0.6
Lianas	300–500 × 10⁻⁴	150	500	approx. 10	0.5

At a constant volume flow, the water potential decreases (i.e. gets more negative) with increasing radius of the vessels. However, the advantage of wide vessels for rapid transport of large amounts of water is counteracted by the increased risk of cavitation (Tyree and Sperry 1989). The forces of cohesion, maintaining a continuous column of water in the xylem, decrease with increasing radius of the xylem vessels (Eq. 10.13) from about 1500 Pa in tracheids of conifers (r = 10–40 μm) to about 60 Pa in tracheids of ring-porous woods (r = 500 μm; Table 10.2). Thus, there is the danger that the cohesive force determining the continuity of the water column is exceeded. Cavitation describes the breakage of the water column in xylem cells. It is caused by small air bubbles, filled mainly with water vapour, forming in a thermodynamically unstable condition (Tyree 1997; Steudle 2000, 2001; Stroock et al. 2014). Once initiated, the bubble expands, causing an embolism in the xylem cell—for example, after injury. During the breakage of the water column, the flux in the vessel is interrupted. In wood, the pits of cell walls that separate xylem vessels seal cavitated vessels off. The water transport is redirected laterally (Grace 1993). Cavitation may be healed by various processes (Holbrook and Zwieniecki 2005). At high water potential, water vapour may condense again, restoring water column continuity. Cavitation and embolism may also be healed via root pressure if the plant is not too tall (Sperry et al. 1987; Gartner 1995). Plants may also be able to refill cavitated xylem vessels by phloem water because of the difference in the water potential in adjacent parenchyma (Hölttä et al. 2006; Nardini et al. 2011). However, if cavitation affects complete organs (leaves or branches), these parts dry and die.

Since the risk of cavitation increases with increasing size of xylem vessels (Fig. 10.15a) (Lo Gullo and Salleo 1991), cavitation occurs first in vessels with a large lumen, while the water column in vessels with a small lumen remains intact, even at high water tension. The structure of the conducting tissue determines the risk of cavitation at high rates of water transport into the shoot (Grace 1993). However, the plant is not unprotected in the face of this danger. With increasing drought, water transport in the soil and root changes, but the stomata will restrict the water flow (Sect. 10.3), and the relation of the leaf area to the xylem conducting area can be regulated by slowing of leaf formation or by shedding of leaves. Loss of productivity and plant mortality have been explained by hydraulic failure (Choat et al. 2012).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig15_HTML.jpg — Fig. 10.15
Cavitation in xylem vessels. a Dependence of cavitation in xylem vessels of different size on the water potential in the xylem as the soil dries over several days. At a water potential of approximately −3.3 MPa, all large vessels of *Ceratonia siliqua* (>60 mm), but only 10% of the small vessels (<10 mm), are cavitated (Lo Gullo and Salleo 1991). b Cross-section of *C. siliqua*, consisting mainly of thick-walled fibres. Vessels with variable lumina are arranged in short radial rows. Large vessels in the older part of the wood are closed by thylosis, which are formed by cavitation during drought, whereby vessels that no longer participate in the water flow are sealed. The border of a growth ring is hardly visible, as indicated in the maritime climate of the island of Cyprus, where the investigated stem of *Ceratonia* grew. Fi fibre, Pa parenchyma, *PsPlug* phenolic substance plug, Ra ray, Tr tree ring boundary, Xv xylem vessel. (anatomical section by F. Schweingruber)

Obviously, species “adapted” to a habitat have generally evolved mechanisms to avoid lethal stress situations. Other species would not flourish in these habitats or would restrict their growth and reproductive phase to a short period in which this critical situation does not occur. For example, the Mediterranean Bromus spp. are successful invasive species in North American prairies and Australian semi-deserts, where the vegetative growth is restricted to the period with sufficient water supply. The invasive Bromus spp. gain this water with shallow roots from the top layers of soil at the cost of the water supply to indigenous perennial dwarf shrubs, particularly Artemisia tridentata in North America and Atriplex spp. in Australia (West and Young 2000) (Chaps. 13 and 20), which have deep-reaching root systems.

Generally, the diameter of vessels is larger in roots than in stems of the same species (Martínez-Vilalta et al. 2002), further decreases in peripheral organs like branches and twigs, and is particularly small in the petioles of leaves. While water potentials in the xylem decrease with increasing distance to the soil within the water conduits, the danger of cavitation is increasing. However, the small vessel diameters in the petioles reduce the risk of progressive cavitation.

At constant hydraulic conductivity, the water potential in the leaf decreases linearly with increasing transpiration. This can be used to demonstrate structural differences in the stem between species (Fig. 10.16a) (Schulze and Chapin 1987). Plants with lower xylem conductivity (i.e. with a steeper slope in volume per time and unit area and water potential gradient) have lower rates of transpiration and more negative water potentials. In contrast, plants with high xylem conductivity (i.e. with a flatter slope) also have high rates of transpiration; however, the water potential does not decrease to the same extent. In these species, the risk of cavitation is particularly great, leading to a substantial change in conductivity when the soil dries out (Fig. 10.16b) (Schulze and Hall 1982). In these latter species, the xylem water transport under moist conditions takes place in vessels with large lumina, while under dry conditions, xylem water transport is restricted to vessels with narrow lumina, which cavitate rather late (Fig. 10.15b). Regulation of water flux via structural characteristics of the shoot is thus possible and is dependent, to some degree, on the conditions under which a species grows.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig16_HTML.png — Fig. 10.16
Relationships between water potential in the xylem and transpiration a for different plant functional types (Schulze and Chapin 1987) and b for the crop plant *Vigna unguiculata* in drying soil (Schulze and Hall 1982). An increasing transpiration rate causes a lowering of the water potential. The slope of the graph is a measure of the hydraulic conductivity of the plant–soil system. *Circles* and *triangles* show the two groups of plants measured. (see also Fig. 10.10)

The role of the xylem structure in water transport is demonstrated by measurements of water flow in the xylem of pines of different ages in the continental climate of central Siberia (Fig. 10.17) (Zimmermann et al. 2000), where the xylem flow increases linearly with the sapwood area. The greatest sapwood area is achieved at the age of 60 years. At this age the growth rate of trees is relatively high and the formation of heartwood has not yet started. In very old pines the sapwood area decreases, and thus the flow of water decreases. A certain plasticity in xylem development during radial growth according to the water supply has also been described (Eilmann et al. 2010; Plavcová and Hacke 2012; Bryukhanova and Fonti 2013).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig17_HTML.png — Fig. 10.17
Relation between canopy transpiration and stand sapwood area. a The rate of water flow rises linearly with the sapwood area; note that it is neither the youngest nor the oldest and largest trees that have the highest transpiration rates, but the 60-year-old pole stand. The small area of sapwood in the oldest stand is caused mainly by the low tree density in older stands (Modified from Zimmermann et al. (2000)). b *Pinus sylvestris* woods on Pleistocene sand dunes in central Siberia at 60°N on the west bank of the Jennesey. Here, cohorts of even-aged trees establish after fire (see Wirth et al. (1999)). The vegetation of the forest floor is *Cladonia*. (Photo: E.-D. Schulze)

10.2.3 Phloem Water Transport

The importance of water transport in the xylem and the danger of cavitation for the functioning of the whole plant are fully appreciated only when considering also the mass flow of material in the phloem (phloem water transport). In vascular bundles of plants, almost all neighbouring cells transport larger amounts of water in opposite directions, in the xylem from root to shoot, in the phloem from source to sink and laterally from the phloem to all heterotrophic living cells in the stem. Water transport occurs in the xylem along a water potential gradient, and in the phloem along an osmotic pressure gradient (the pressure gradient hypothesis; Fig. 10.18). An additional bidirectional flow of water exists within the meristem (Aloni 2004), the cell layers between the xylem and phloem. The meristematic water flow is important in regulating growth.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig18_HTML.png — Fig. 10.18
Plant-internal water circulation. The driving force is the gradient of water potential, Ψ, which develops as a result of transpiration. In the phloem, the mass flow gradient is a result of the turgor (P), caused by loading and unloading of osmolytes (sugars and ions) in the phloem, and hence the osmotic pressure (Π) changes. In trees, there is the additional hydrostatic pressure (ρ _wgh) to be taken into account, where ρ _w is the density of water, g is gravity and h is tree height. Note: The turgor pressure in the xylem (P^xylem) is negative, unlike in most living cells, due to their different cell wall structure

Different gradients are maintained as follows. In moist soils, the water potential of roots is similar to that of the soil (Ψ_root = 0). With the uptake of ions, the osmotic pressure increases to about 0.1 MPa. Water transport starts because of the decrease of water potential in the leaf (Ψ_leaf = −0.8 MPa) as a consequence of evaporation and is dependent on the vapour pressure of the external air (e) and the net radiation (R _n). In the leaf, a smaller part of this water mass is redirected into the phloem where, because of the additional loading of the sieve elements with cations and sugars, an osmotic potential of 1.7 MPa at a water potential of −1.0 MPa and a turgor pressure of 0.6 MPa develops. This turgor pushes the phloem water from the leaf to the root as a sink. As the tissues along the sieve tubes remove sugar, the hydrostatic pressure decreases with decreasing osmotic pressure. This pressure-dependent transport occurs in an opposite direction to the longitudinal gradient of the water potential between shoot and root. In the root or along the stem, phloem water may re-enter the xylem. This plant-internal water circulation of water occurs even if the plant grows under extremely moist conditions (i.e. the water potential gradient = 0) (Tanner and Beevers 1990) or under very dry conditions. Under these extreme conditions, the circulation of water in the plant may be regarded as a “phloem-driven xylem flux” and may lead to “bleeding” of cut stems by root pressure (e.g. in sugar maple) (Cirelli et al. 2008). This transport would be interrupted only if the complete xylem were not functioning, because of cavitation during drying. In this case, the survival of the plant would no longer be possible. Consequently, all species with large xylem vessels also make small-xylem vessels so that redirection of water flows is possible in the case of cavitation, and special structures (pits) ensure that the damage is limited—that is, that cavitation does not continue in the stem. In the case of drying out, not only the water potential changes but also the osmotic pressure changes so the pressure gradient in the phloem is maintained (Schulze 1993).

10.3 Transpiration

Evaporation of water out of leaves (transpiration) starts at the surfaces of cell walls lining the intercellular spaces, from where the water vapour, as a consequence of the vapour pressure gradient, reaches the external air by diffusion via the stomata. The stomata are the valves limiting diffusion, regulated by processes in the leaf and in the roots, and by conditions in the environment (Fig. 10.2).

The guard cells of the stomata are a pair of cells, which stick together only at the ends, leaving a gap—the stomatal pore—in between (Fig. 10.19) (Meidner and Mansfield 1968).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig19_HTML.jpg — Fig. 10.19
Anatomy of guard cells. a Schematic structure of the cell wall of guard cells of dicotyledonous plants (*left*) and monocotyledonous plants (*right*). Micelles in the cell wall (*hatched lines*) are arranged so the expansion of the cell with increasing turgor can occur only perpendicularly to their orientation (from Meidner and Mansfield (1968)). b Scanning electron micrograph of a guard cell of grapevine (*left*) and of wheat (*right*). (Troughton and Donaldson 1972)

The stomatal pore can open whenever the guard cells expand with increasing turgor. Because of the orientation of the micelles in the cell wall, the cell volume changes mainly by longitudinal expansion perpendicular to the orientation of the micelles. Thus, both cells form an opening of different widths (stomatal aperture), depending on the turgor pressure in these cells. The stomatal density (number of stomata per unit of epidermal area) and the size of the aperture depend on the species and site conditions, even though general relations do not emerge (Meidner and Mansfield 1968). Thus, the number of stomata (per square millimetres of leaf area) varies between 30 (in Triticum spp. and Larix spp.) to more than 5000 (in Impatiens). The size of the stomatal pore varies between 77 × 42 μm (in Phyllitis spp. and Tradescantia spp.) and 25 × 18 μm (e.g. in Tilia spp.).

The opening mechanism is based on a physiologically regulated change of turgor, where K⁺ ions are taken up from the neighbouring cells (the so-called subsidiary cells). This ion uptake is regulated by the membrane potential (Chap. 6). The movement of the stomatal aperture is asymmetrical—that is, closing occurs much faster (1–10 min) than opening (30–60 min) (Lange et al. 1971).

Measurement of the stomatal apertures is possible in nature in some species with a microscope (Kappen et al. 1994). However, for most plant species, the aperture cannot be seen, as it is covered by protrusions of the cuticle or by waxy scales. Direct observations of the aperture are also difficult because disturbance of the leaf and the surrounding climatic conditions may affect the measurement. Therefore, rather than carrying out direct observations, in an analogy to Ohm’s law the leaf resistance (R _L) or the leaf conductance (g _L) are calculated from the transpiration flux (E _L) and the gradient in the vapour pressure between the leaf and the air (D _L; where _L refers to the leaf as a whole):

${g}_{\mathrm{L}}=\frac{E_{\mathrm{L}}}{D_{\mathrm{L}}}=\frac{1}{R_{\mathrm{L}}}$

(10.15)

where the dimension (in mmol per square metre per second) has the same dimension as transpiration. At 15 °C, a conductance of 1 mmol m⁻² s⁻¹ corresponds to a conductivity of 4.24 mm s⁻¹ (3.83 mm s⁻¹ at 45 °C). Equation 10.15 neglects cuticular transpiration, which is low for most plants. In general, stomatal conductance (Eq. 10.16) is preferred to resistance to estimate transpiration, as it changes in proportion to the flux (Cowan 1977):

${E}_{\mathrm{L}}={D}_{\mathrm{L}}{g}_{\mathrm{s}}$

(10.16)

The volume flow (i.e. transpiration (E _L)) is determined by meteorological conditions in the atmosphere and by the stomatal aperture (Eq. 10.16). The vapour pressure of the air determines a directed diffusion of water molecules from the mesophyll via the stomata to the atmosphere. This transpirational water flow lowers the water potential in the leaf, which in turn results in a liquid flow of water in the xylem, which may feed back on stomatal opening. In this feedback loop, stomatal conductivity in plants is tuned to the hydraulic conductivity of the stem (see, for example, Hubbard et al. (2001)). Particular hydraulic barriers occur in grasses, where water passes through parenchyma at the nodes.

The stomatal aperture is regulated by environmental conditions related to climatic factors (so-called feed-forward regulation) and by processes in the mesophyll (so-called feedback regulation). Feedback regulation is determined by water status and CO₂ assimilation. Fig. 10.20 illustrates the different responses, which are described in detail in Sects. 10.3.1 and 10.3.2.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig20_HTML.png — Fig. 10.20
Schematic model of the stomatal response to the environment and plant-internal factors. A “root signal” may be a change in water potential or a hormonal signal. ψ water potential, c _a atmospheric CO₂ concentration, c _i CO₂ concentration in the mesophyll, D _L water vapour pressure deficit between leaf and air, E _L transpiration, g _CO ₂ stomatal conductance of CO₂, g _H ₂ _O stomatal conductance of H₂O, g _max maximum stomatal aperture (a measure of long-term responses to stress), g _s variable stomatal conductance

10.3.1 Stomatal Responses to Plant-Internal Factors

Photosynthetic capacity: The maximum opening of stomata (g _max) correlates with the capacity for photosynthesis of leaves, depending on the nitrogen supply—which affects the carboxylation capacity (Schulze et al. 1994) (Chap. 12) and the specific leaf area (SLA, leaf area/leaf weight)—and on the species (greater in fast-growing species) (Reich et al. 1997). From the relation between conductance and leaf nitrogen concentration, global maps of maximum stomatal conductance are produced, from which also maximum rates of CO₂ assimilation can be derived (Chap. 12, Fig. 12.6c). The maximum capacity for the stomatal opening of a given plant has been described only empirically.

Soil water status: The availability of water in the soil is globally the most important environmental factor limiting stomatal conductance. During soil drying, a “root signal” regulates stomatal conductance. This root signal could be a hormonal signal (e.g. ABA) or a hydraulic signal (Heilmeier et al. 2007) (Chap. 6).

It has often been observed that stomata close with decreasing water potential in the leaf. However, the leaf water potential changes with transpiration as well as with the water supply from the soil (Fig. 10.21). This makes it difficult to separate atmospheric effects on transpiration from those of soil water status. Experimentally, both effects can be separated by compensating the matrix potential of the drying soil with a hydrostatic pressure applied to the soil. Thus, the effect of soil drying can be observed on fully turgid leaves. In this case, stomatal closure is dependent on the soil water content (Schulze 1994).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig21_HTML.png — Fig. 10.21
Effect of leaf and soil water status on leaf conductance. a Relationship between conductance of the leaf and water potential in the xylem in *Nerium oleander* with increasing soil drying. The gradient of water vapour pressure between the leaf (on which the stomatal conductance was measured) and the air around the leaf was constant (10 Pa kPa⁻¹), but the plant itself was either in moist air (*black symbols*) or in dry air (*blue symbols*). The stomata closed (decreasing conductance) as the soil dried, but the humidity of the air around the whole plant altered the leaf water potential by almost 1 MPa in dry air compared with moist air, although the measured leaf was under constant conditions. b Leaf conductance in the same experiment, but related to soil water content (Modified from Gollan et al. (1985)). The results of this experiment imply that the stomata respond not to the leaf water potential but to the soil water content

10.3.2 Stomatal Responses to Environmental Factors

Carbon dioxide: Stomata react to the gradient in the CO₂ concentration between the external air and the intercellular spaces of the leaves (Chap. 12). In a classical experiment, Raschke (1972, 1979) was able to open stomata by decreasing the CO₂ concentration down to a CO₂-free environment and close them by increasing the CO₂ concentration until saturation was reached. Neither light nor darkness influenced the observed CO₂ concentration effects. The CO₂ gradient between the leaf and the air, as expressed by the CO₂ concentration ratio, C _i/C _a, remains the main variable to model photosynthesis.

Epidermal water status: With decreasing turgor in the epidermis, the aperture of the stomata decreases (Nonami et al. 1990). In contrast to the reaction of the leaf to changes in the root water potential, this is a cellular imbalance in the epidermal turgor dependent on transpiration (Schulze 1993).

Light: Stomata open with increasing light intensity. In the morning, stomatal conductance increases earlier than photosynthesis. Therefore, stomata do not limit the uptake of CO₂ during the early morning when the humidity is high.

Temperature: At low temperatures (freezing point), stomata are closed, and they open as the temperature increases. This opening is exponential at temperatures above 40 °C, so the leaf temperature can decrease even below that of the air because of the strong cooling by transpiration if water is available and under low rH values (Chap. 9).

Air humidity and leaf water status: Stomata close with an increasing vapour pressure deficit between the leaf and the air, and this can also be observed with an isolated epidermis (Lange et al. 1971), where closing is faster than opening (Fig. 10.22a). This response can be so strong that transpiration decreases despite an increasing water vapour gradient between the leaf and the air (Fig. 10.22b; Schulze et al. 1972). It is still unclear how stomata “measure” humidity. From measurements with He-enriched air, Peak and Mott (2011) proposed that water vapour is the driving force for the stomatal response to humidity.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_10_Chapter/72100_2_En_10_Fig22_HTML.png — Fig. 10.22
Stomatal response to air humidity. a Stomata in isolated epidermis of *Polypodium vulgare*. The lower surface of the epidermis was in contact with water. Only a small air bubble simulated the sub-stomatal air space. On the upper surface, dry or moist air was blown over the stomata with a capillary. The experiment started with closed stomata in dry air. A change to moist air induced slow stomatal opening within 54 min. With constant moist conditions the stomata stayed open, but upon application of dry air they closed within 4 min (Lange et al. 1971). b Response of measured leaf conductance, transpiration and water content of a *Prunus armeniaca* leaf to dry air. The stomata closed, transpiration decreased and leaf water content increased (Modified from Schulze et al. (1972)). c Responses of different types of plant to dry air. (Schulze and Hall 1982)

In tall trees the reaction to local deficits probably plays an important role in the regulation of stomata during the course of the day. In the canopy of a forest, turbulence of air movement occurs with fast exchanges of air packages with differing humidity. This correlates with fast changes in xylem flow. Neighbouring trees exposed to the same air masses show synchronous changes in xylem flow (Hollinger et al. 1994). Stomatal closure is induced by short-term changes in transpiration and the associated changes in the water state of the epidermis (Köstner et al. 1992). As closure is faster in dry air than opening is in moist air, a continuous decrease in stomatal conductance during the day is the consequence of fluctuating humidity in the atmosphere.

The stomatal response to humidity has been presumed to be a function of the driving force of transpiration, D _L, the gradient of water vapour concentration between the leaf and the air (Eq. 10.15), even though the response to D _L decreases with temperature. Also, with stomatal conductance as defined by D _L, no common relation with CO₂ assimilation can be observed. However, if the response of stomatal conductance to water vapour, g _sw (measured in moles per square metre per second), is scaled to relative humidity at the leaf surface (h _s) and to the mole fraction of CO₂ at the leaf surface (c _s), a linear relation emerges. This includes the response to CO₂ assimilation, A, and to air humidity at different temperatures, with k being an empirical coefficient, 9.31, which may depend on the species (Ball et al. 1987).

${g}_{\mathrm{s}\mathrm{w}}=k\ A\ \left({h}_{\mathrm{s}}/{c}_{\mathrm{s}}\right)$

(10.17)

Equation 10.17 is interesting in view of the biophysics of stomatal regulation, because the relative humidity at the leaf surface would express the water potential at the site of evaporation (see Eq. 10.7), and it would be an expression of the “hydration of the evaporating surfaces in the mesophyll”, which could regulate stomata. Pieruschka et al. (2010) suggest that the driving force for all stomatal responses to the environment is radiation, which controls the water vapour production in the leaf interior.

10.4 Summary

This chapter covers a wide range of plant organisation, starting from cellular water relations and ending with the regulation of water flow at the leaf level. (The responses of canopies and landscapes are discussed in Chap. 16.)
Water vapour is the most abundant greenhouse gas in the atmosphere, which affects not only the climate via infrared absorption but also plant life via precipitation and humidity. The ratios of H₂O to CO₂ and H₂O to O₂ are the reasons why animals living on land have more favourable water use during oxygen exchange for respiration (about 1:1) than plants do during CO₂ exchange for photosynthesis (about 200:1).
Plant water storage to support transpiration is not possible in the long-term for most species. Desiccation-tolerant (poikilohydric) plants and desiccation-intolerant (homoiohydric) plants represent different strategies to cope with dry atmospheres. Poikilohydrics are competitive at high dewfall and low precipitation (lichens in coastal deserts).
Water is not freely available but bound by chemical, capillary and osmotic forces (the matrix and osmotic components of water potential), where the water potential describes the state of water in homogeneous and heterogeneous systems. It is a measure of the energy necessary to convert bound water to the state of free water. Water flows from a compartment with higher water potential to one with lower water potential. Cells regulate their water status by altering the osmotic pressure in the vacuole and thus maintain turgor in a quasi-steady state. Plant availability of water depends on the soil texture. Hygroscopically bound water is water that is bound by forces >5 MPa. The permanent wilting point is defined as a soil water status of −1.5 MPa when a sunflower plant is no longer able to replace the water that is lost through transpiration from the water in the soil, and thus wilts. The field capacity is defined as water bound at −0.05 MPa when water can no longer be retained by soil particles against gravity and therefore drains out of the soil. The amount of water in the soil available for plants corresponds to the water content between the field capacity and the wilting point.
Plants exploit water with roots of very different anatomy and morphology. Water uptake is constrained to the unsuberised root and to meristematic tissue of lateral roots. By the hydraulic and inverse hydraulic lift, water may be redistributed in the soil profile.
With water transport in the xylem, the flow of water increases with increasing size of vessels at increasing risk of cavitation, and by the number of vessels involved in transport, the functional xylem area increases. Xylem conductance is smallest in conifers and increases progressively in diffuse and ring-porous woods, herbaceous plants and lianas.
Xylem transport occurs along a water potential gradient and is dependent on the vapour pressure deficit of the air, where the greatest change in potential occurs between the mesophyll and the atmosphere, where stomata regulate the water loss. Phloem transport occurs along a gradient of osmotic pressure, which depends on the loading and unloading of sugars. Circulation of water within the plant between the phloem and xylem is important for plant survival. Regulation of the xylem flux is achieved by stomatal closure, leaf abscission, partial senescence and hardwood formation.
Stomata are the most important valves by which plants can regulate transpiration, which is a physically determined process that is under physiological regulation within certain limits. Since the stomatal aperture cannot be observed directly in most species, it is expressed and measured as “conductance” in an analogy to Ohms law. Stomata regulate the flux of CO₂ into the leaf for photosynthesis, they respond to root signals during drought, and they respond to environmental factors, particularly light, temperature and atmospheric humidity.

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