INTEGRATION OF HYDRAULIC AND CHEMICAL SIGNALING IN THE CONTROL OF STOMATAL CONDUCTANCE AND WATER STATUS OF DROUGHTED PLANTS

We describe here an integration of hydraulic and chemical signals whic h control stomatal conductance of plants in drying soil, and suggest t hat such a system is more likely than control based on chemical signal s or water relations alone. The determination of xylem [ABA] and the s tomatal response to xylem [ABA] are likely to involve the water flux t hrough the plant. (1) If, as seems likely, the production of a chemica l message depends on the root water status (PSI(r)), it will not depen d solely on the soil water potential (PSI(s)) but also on the flux of water through the soil-plant-atmosphere continuum, to which are linked the difference between PSI(r) and PSI(s). (2) The water flux will als o dilute the concentration of the message in the xylem sap. (3) The st omatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water-flux-dependent message, with a water-f lux-dependent sensitivity. In such a system, we have to consider a com mon regulation for stomatal conductance, leaf and root water potential s, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the genera tion and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the sol ution suggested that such common regulation can operate. Simulations s uggest that, as well as providing control of stomatal conductance, int egration of chemical and hydraulic signalling may also provide a contr ol of leaf water potential and of xylem [ABA], features which are appa rent from our experimental data. We conclude that the root message wou ld provide the plant with a means to sense the conditions of water ext raction (soil water status and resisance to water flux) on a daily tim escale, while the short-term plant response to this message would depe nd on the evaporative demand.