VARIABILITY AMONG SPECIES OF STOMATAL CONTROL UNDER FLUCTUATING SOIL-WATER STATUS AND EVAPORATIVE DEMAND - MODELING ISOHYDRIC AND ANISOHYDRIC BEHAVIORS

Stomatal control of species with contrasting stomatal behaviours have been investigated under natural fluctuations of evaporative demand and soil water status. Sunflower and barley (anisohydric behaviour) have a daytime leaf water potential (psi(1)) which markedly decreases with evaporative demand during the day and is lower in droughted than in wa tered plants. In contrast, maize and poplar (isohydric behaviour) main tain a nearly constant psi(1) during the day at a value which does not depend on soil water status until plants are close to death. Plants w ere also subjected to a range of soil water potentials under contrasti ng air vapour pressure deficits (VPD, from 0.5 to 3 kPa) in the field, in the greenhouse or in a growth chamber. Finally, plants or detached leaves were fed with varying concentrations of artificial ABA. Stomat al conductance of well-watered plants had no response to VPD when plan ts were grown in natural soils, suggesting that the opposite result ob served in many laboratory experiments might be linked to the low unsat urated hydraulic conductivity of usual potting substrates. The respons e of stomatal conductance of all studied species to the concentration of ABA in pressurized xylem sap ([ABA](xyl)) was the same whether ABA had an endogenous origin (droughted plants) or was artificially fed. H owever stomatal response of maize and poplar to [ABA](xyl) markedly ch anged with varying evaporative demand or psi(1), whereas this was not the case in sunflower or barley. This suggests that isohydric behaviou r is linked to an interaction between hydraulic and chemical informati on, while anisohydric behaviour is linked to an absence of interaction . In all cases, [ABA](xyl) was related to soil water status with commo n relationships for different experimental conditions, but with marked ly different responses among species. Diurnal variations of [ABA](xyl) with evaporative demand were small in all studied species. Results ar e synthesized in a model which accounts for observed behaviours of g(s ), psi(1) and [ABA](xyl) in fluctuating conditions and for several spe cies. The validity of this model, in particular the physiological mean ing of [ABA](xyl), is discussed.