10 20 30 40
Soil water content (%)
Fig. 2.2.4. A The change in water potential in a sandy and a loamy soil with increasing water content. Depending on texture the water potential at field capacity is -0.05 MPa (sand) and -0.015 MPa (loam). Convention sets the permanent wilting point 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 as -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 1976). B Change in the amount of water available to a plant with increasing rainfall on a loam and sandy soil. In arid areas sandy soils contain more water that is available to a plant than a loam soil as the water is not so strongly held by capillarity in sand as it is on loam. In contrast, in areas of high rainfall, the amount of water that can be stored by a sandy soil is less than a loamy soil. The shaded areas indicate the rainfall conditions that are most suitable for plants for that soil type (from Walter 1960). CThe distribution of vegetation in the Namib Desert with Acacia karroo, Aristida sabulicola and Acacia ciliata growing on the sand dunes (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 concentration. Asab, Namibia. (Photo E.-D. Schulze)
draulic conductivity, which depends on the water content of the soil. In dry soils the limiting factor is the conductivity of the unsaturated soil where water moves in the vapour phase. In contrast, in saturated moist soils, the limiting step for water uptake is the conductivity of roots (the latter is usually the case in temperate climate zones). Depending on texture and saturation of the soil, roots must grow towards the water or "wait" until the water flows from the soil to the root. This determines the surface area of roots required to provide leaves with water.
Water transport in soils is differentiated into saturated (water content above field capacity) and unsaturated states (water content below field capacity). According to Darcy's law (1856), based on studies of water pressure in the wells of Dijon, the rate of flux (v) in saturated soils 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 conductivity of the soil (ks), v = —ks(dh/dx) (2.2.11)
where ks depends on the particle size. In stony soils ks is > 0.001 ms-1, in sandy soils >10~5 m s_1, in silty soils <10~7ms_1 and in clayey soils < 10"9 m s"1.
In unsaturated soils, the rate of flux is much smaller than in saturated soils; it depends on the unsaturated conductivity, k0, which decreases with soil water content, 0, and is determined by the potential gradient, A W, over the distance, x.
This equation is analogous to Eq. (2.2.11), but for unsaturated soils the water potential is the driving force. The values for k0 in silty soils is between 10~13 (saturated soils) and 10~17cms_1 Pa-1 in the range of 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 soil crumbs in close proximity may decrease over a very short distance with the soil water content (Horn 1994). Also, within a soil profile, water is available to very different degrees.
It is not surprising that plants develop many different forms of root systems, considering the heterogeneity of water status and flow in soils and the modifications which depend on soil texture. Grasses develop a particularly dense adventitious root system near the soil surface. Dicotyledons, in contrast, form a less extensive root system, but penetrate the ground much deeper with their very large primary root. In horizons where nutrients and water are easier available they are also able to form secondary roots.
The soil layer from which a plant gains its transpiration water may be determined with stable isotopes of hydrogen, D = deuterium, and oxygen, 180. The isotope relation of H/D (the r)D value) and of 160/180 (the <S180 value) increases with the temperature of precipitation (hydraulic water line; Dawson 1993), i.e. winter rains have a different isotopic signal than summer rains. In addition, the heavier isotope accumulates at the soil surface, as water molecules with the heavier isotope evaporate slower than molecules with the lighter isotope. Measuring the isotope rates in xylem water and in soil water provides information on the horizons from which certain species get their water.
It is, of course, easier to establish the origin of transpiration water with reference to the soil depth in regions with seasonal rainfall at different temperatures. An example is given in Fig. 2.2.5 A for the Colorado Desert in the southwestern USA (Ehlinger 1994). Summer and winter rains show very different r)D values and groundwater is even more depleted of deuterium than winter rain. Various plant species use water sources from different depths. In this subtropical "warm" desert (Fig. 2.2.5 B), annuals and succulents use summer rains, which reach the Sonora 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 over-supply of water there are longer dry periods where these plants lose their leaves, i.e. they are deciduous. In the ecological literature, a distinction is made between arido-active plants which keep their photosynthetically active leaves during the dry period, and arido-passive plants which either shed their leaves or minimise metabolism. Figure 2.2.5 shows that there are many
Winter rain Well water
Winter rain Well water
| Fig. 2.2.5. A Deuterium isotopic ratio in water from xy-lem of different types of plants growing in the Sonora Desert, compared with the deuterium ratio in summer and winter rainfall, as well as in ground water (after Ehleringer et al. 1994a,b). The <5D value is calculated from D/H of the sample in comparison with a standard [(D/H)samp|e/ (D/H)standard-1]x1000, where water from deep oceans is used as the standard (SMOW=standard mean ocean water). In the case of the Sonoran Desert, annuals use the summer rain water almost exclusively, which has a high <5D value dependent on the temperature. In contrast, deep-rooted perennials almost exclusively use water from winter rainfall with a low <5D value due to the lower temperatures. The <5D 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 paposa (Asteridae, C4 plants), perennial woody plants, Ambrosia dumosa (Asteridae, C3 plant) and perennial deep-rooted Larrea tridentata (Zygophyllaceae). (Photo E.-D. Schulze)
Deep rooted perennials
transitions which make such a classification (as appears so often in the older ecological literature) unsuitable. The differentiation of species according to their source of water not only applies to the Colorado Desert, but also to other dry regions, e.g. the temperate semi-deserts in Argentina (Schulze et al. 1996 c).
Water uptake from the soil into a plant occurs at a zone behind the apex of root tips where root hairs develop and the root cortex is not yet su-berised (Steudle 1994). In addition, water is taken up at meristematic regions of the side roots. Above the meristematic region, roots are differentiated (Fig. 2.2.6) into an epidermis, in many species a suberised layer called the exodermis, followed by the root cortex, the heavily suberised endodermis (Casparian bands) and the central cylinder (stele) with the xylem vessels and the phloem. Within the root cylinder, water follows (as a first approximation) the water potential gradients from the soil to the xylem, but it may follow several pathways (Fig. 2.2.6 B). In the region of the root cortex: (a) water may flow in the cell wall (apoplast), (b) move from cell to cell via the plasmodesmata (symplast), or (c) across the cells (transcellular path). At the endodermis (and probably also at the exodermis) water must be moved through the cell. Unsuberised transmission cells exist in this region. Water transport in the root may be explained by a model with a series of parallel resistances connected at regular intervals by serial resistors. The hydraulic conductivity in each cell layer is the conductivity of a non-cylindrical tissue (Lpz).
| Fig. 2.2.6. A Cross section through a maize root in which the lignin and lipids in the exodermis and endoder-mis are stained with berberin sulfate. 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)
Casparian band Cortex
Casparian band Cortex
Lpc Lpcw Ax
where Lpz is the conductivity of a cell in m s_1 MPa-1. Lpc describes the conductivity of cell membranes, where the factor 2 considers the fact that two membranes per cell must be crossed. Lpcw is the conductivity of the cell wall, yc and ycw the amount of cytosol and cell wall at the cross section in the direction of the flux (Ycw+Yc= 1), Ax the width of a cell layer and d is the thickness of the tissue. This shows that the relevant fluxes may be differently distributed according to the structure of the root cortex. The distribution of pore size in the cell wall and the size of the hydrated ions determine the conductivity. Pores in the intermicella space are about 1 nm; the space between the cellulose fibrils is about 10 nm. In comparison, a water molecule is about 0.3 nm, Na ions with their hydrated shell reach 0.5-0.7 nm, K ions 0.4-0.5 nm and a glucose molecule is about 0.75 nm (Liittge 1973). Even though the cross section available for apoplastic transport is much smaller than that for cellular components, measurements of the components have shown that the apoplastic transport dominates when the flux is hydraulic (transpiration suction) and does not follow the osmotic gradient. If the apoplastic transport path is effectively interrupted (strong suberisa-tion of roots), the cellular component dominates (Michael et al. 1997). The flow of water through the cell membrane in the cellular transport path is actively regulated by aquaporins (proteins, which transport water through the otherwise hydrophobic cell membrane: water channels; Tyer-man et al. 1999; see Chap. 1.5.2). In strongly differentiated regions of roots and less differentiated regions, water may be taken up in parallel; water flow is also possible in the suberised regions of the root, namely in places where side roots are formed, in the edge between the main and the side root meristematic tissues are retained. The root is thus not a uniform surface vis-à-vis the soil, but a very differentiated region of uptake, called by Steudle and Peterson (1998) a compound membrane. Water uptake supports the water status of the plant (ï^hoot) which, however, depends not only on water uptake -the input - , but also on transpiration - the output -; both are separately regulated but tuned to each other by hormonal and pressure signals.
It has been known for more than 60 years that the hydraulic resistance of the root, and thus the water uptake, is variable. In contrast to the shoot and its regulation of water loss via stomata (see Chap. 2.2.3), the regulation of the water uptake is also via the hydraulic architecture of the roots and, because of technical limitations, this is much less studied than the hydraulic architecture of the shoot. Also, processes in the root are not obvious. Regulation appears as a purely physical adaptation to water uptake as well as by metabolic regulation.
The differentiation in root anatomy is, from an ecological point of view, an adaptation of plants to the conditions of water flow between soil and root and the associated flow of nutrients. The process of 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 the transpiration and may fall below a critical value for metabolic processes in the leaf even at wet sites. In moist soils the water potential in the xylem sinks with the amount of water that is transported through the system (Fig. 2.2.7, curve A). 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, i.e. further supply from the soil may, in this case, be the limiting factor for the flow of water (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. 2.2.7, curve B). A corresponding turgor loss is expected to occur in the root tip under such conditions, leading to the production of the stress hormone ABA (see Chap. 220.127.116.11). Conversely, there are situations where changes in water transport occur without changes in water potential (Fig. 1.1.7, curve C). This observation is described by the transport model in Fig. 2.2.6. Here, the water potential gradient between soil and xylem reaches a magnitude which allows a flux via additional surfaces, i.e. other roots or root regions which were not participating in water uptake because the potential gradient was too low.
The sites for water uptake also harbour a danger, in as far as the plant is not able to totally seal itself against the soil. The unprotected
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