Evaporation of water from plants (transpiration) takes place 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. Stomata are the regulatory valves limiting diffusion. These valves are regulated by processes in the leaf, as well as by processes in the roots, and by conditions in the environment (see Fig. 2.2.1).
Guard cells of stomata are structured in pairs which only grow together at the ends (Fig. 2.2.14; Meidner and Mansfield 1968). The middle part, which is not fused, forms an aperture. This opens to a varying extent because the expansion of the cell with increasing turgor is radially restricted due to the orientation of the micelles in the cell wall. Thus the cell volume changes mainly by longitudinal expansion, and, together with the counter pressure of the surrounding epidermal cells, both cells form an opening of different widths (aperture). The number of stomata per unit epidermal area and the size of the aperture are, to a large degree, species and site specific (Meidner and Mansfield 1968). Thus the number of stomata (mm-2 leaf
• Availability of water depends on the soil texture. This determines the amount of bound water and conductance of water in the soil.
• Plants exploit water with the formation of roots of very different anatomy and morphology.
• Water flux in roots depends on external conditions, as well as on root structure both in the symplast and the apoplast. Water uptake is not constrained to the root tips, it can also occur in areas of lateral roots where there is meristematic tissue.
• Limited water transport in the soil can lead to deficits in the rhizosphere. Roots can transfer water to the soil if part of the root system is in soil horizons with higher water potential; this leads to the phenomenon of hydraulic and inverse hydraulic lift which re-distribute water in the profile.
• With water transport in the xylem, the flow of water increases with increasing size of vessels but at the same time the risk of cavitation increases.
• Water transport and the risk of cavitation are determined by alterations in xylem radius, the number of vessels involved in area) varies between 30 (Triticum, Larix) to more than 5000 (Impatiens). The size of stomata varies between 77x42 (.im (Phyllitis, Tradescan-tia) to 25x18 (.im (e.g. Tilia). The opening mechanism is based on a physiologically regulated change of turgor, where K+ ions from the neighbouring cells (the so-called subsidiary cells) and the cell wall are taken up. This ion uptake occurs in exchange with H+ ions, where the protons are released from malate, and this acid metabolism is connected with the degradation of starch. The movement of the stomatal aperture is unsymmetrical, i.e. closing occurs much faster (1-10 min) than opening (30-60 min; Lange et al. 1971).
Measurement of the apertures of stomata is possible with a microscope (Kappen et al. 1994), even though for many plant species the aperture is covered by protrusions of the cuticle or by waxy scales. In the field, these investigations are very difficult because of disturbance of the leaf and of the climatic conditions around it. There-
transport and the differentiation of vessel size in the shoot.
• Conductance of the xylem is smallest in conifers, and increases progressively in diffuse and ring porous woods, herbaceous plants and lianas.
• The maximum longitudinal water flow is limited by cavitation in vessels and is primarily dependent on the cross-sectional area of the stem occupied by functional xylem area (=cumulative cross-sectional area of xylem to stem cross section).
• Xylem transport occurs along a water potential gradient and is dependent on the vapour pressure deficit of the air.
• 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 phloem and xylem is important for plant survival, i.e. it is maintained even under unfavourable environmental conditions. Regulation depends on xylem flux, and is achieved by stomatal closure, leaf abscission and partial senescence (crown drying in oak). Regulation of phloem transport occurs through the osmotic pressure.
fore, rather than carrying out direct observations, analogous to Ohm's law a resistance (RL) or a conductance (gL) are calculated from the transpiration stream (EL) and the gradient in the vapour pressure between leaf and air (DL) when the subscript L refers to the leaf as a whole:
with the dimension (mol m~2 s_1), conductivity has the same dimension as transpiration. At 15 °C, conductivity of 1 mol m~2 s_1 corresponds to a conductivity of 4.24 m s-1 (3.83 m s-1 at 45 °C). This scheme neglects cuticular transpiration, which is very low for most plants. In general conductivity is used as the standard, as it changes in proportion to the flux:
Stomatal aperture is regulated directly by environmental conditions related to climatic fac
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