INFLUENCE OF LEAF WATER STATUS ON STOMATAL RESPONSE TO HUMIDITY, HYDRAULIC CONDUCTANCE, AND SOIL DROUGHT IN BETULA-OCCIDENTALIS

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g(S)) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (Delta N; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Psi(S)) As expected, g(S) dropped in response to decreased k or Psi(S), or increased Delta N (> 0.025). Field data showed a decrease in mid-day g(S) with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Psi from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g(S) were associated with approximately constant bulk leaf water potential (Psi(L)), this does not logically exclude a feedback response between Psi(L) and g(S). To test the influence of leaf versus root water status on g(S), we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased Delta N, decreased k, and decreased Psi(S) was fully or partially reversed within 5 min of pressurizing the soil. Bulk Psi(L) remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When Delta N was low (i.e., < 0.025), pressurizing the soil either had no effect on g(S), of caused it to decline; and bulk Psi(L) increased. Increased Psi(L) may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high Delta N, low k, and low Psi(S). Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.