Fig. 2.2.16. 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 of the air around the leaf was constant (10 Pa kPa-1) but the plant itself was either in moist (black symbols) or dry (blue symbols) air. The stomata close (decreasing conductance) as the soil dries, but the humidity of the air around the whole plant alters the leaf water potential almost 1 MPa in dry 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 (after Schulze 1986). This experiment shows that the stomata do not respond to xylem water potential but to soil water content regulate the turgor pressure of guard cells. This is called a root signal, as part of the root-shoot communication occurring in dry soils via the production of ABA (Schroder et al. 2001). Stomata close with increasing ABA concentration in the xylem stream. This reaction is modulated by the high pH in chloroplasts, which are a strong sink for ABA. Apparently, the pH value in the cell wall determines whether ABA is immobilised in the mesophyll and metabolised, or whether it reaches the epidermis. Here also a steady state is achieved, determined by the breakdown of ABA in light and by the transport of ABA in the phloem. With ample water supply, and good nutrition, roots produce another plant hormone, cytokinin, which opens stomata, in contrast to ABA (see also Chap. 1.5, Stress physiology).

It has often been observed that stomata close with decreasing water potential in the leaf.

However, this correlation does not describe the basic mechanism, as the leaf water potential depends on the rate of transpiration, as well as on the state of water in the soil and the supply of water (Fig. 2.2.16). Stomatal conductance increases with the availability of water in the soil and decreases when the soil dries out (Fig. 2.2.16 A; Schulze 1994). Parallel to transpiration and water supply, the leaf water potential changes as well. Thus, the effects of the influence of water availability in the soil are masked (Fig. 2.2.16 B). The effect of the root signal can be verified by compensating the matrix potential of the drying soil by a hydrostatic pressure applied to the soil. Thus the effect of soil drying may be observed on fully turgid plants. In this case stomatal closure is also dependent on the soil water content (Schulze 1994). However, this does not mean that stomata do not also react to the turgor in the leaf (see below).

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