Variability among species of stomatal control under fluctuating soil water status and evaporative demand: modelling isohydric and anisohydric behaviours

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 (ΨI) which markedly decreases with evaporative demand during the day and is lower in droughted than in watered plants. In contrast, maize and poplar (isohydric behaviour) maintain a nearly constant ΨI during the day at a value which does not depend on soil water status until plants are close to death. Plants were also subjected to a range of soil water potentials under contrasting 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. Stomatal conductance of well-watered plants had no response to VPD when plants were grown in natural soils, suggesting that the opposite result observed in many laboratory experiments might be linked to the low unsaturated hydraulic conductivity of usual potting substrates. The response 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. However stomatal response of maize and poplar to [ABA]xyl markedly changed with varying evaporative demand or ΨI, whereas this was not the case in sunflower or barley. This suggests that isohydric behaviour is linked to an interaction between hydraulic and chemical information, while anisohydric behaviour is linked to an absence of interaction. In all cases, [ABA]xyl was related to soil water status with common relationships for different experimental conditions, but with markedly different responses among species. Diurnal variations of [ABA]xyl with evaporative demand were small in all studied species. Results are synthesized in a model which accounts for observed behaviours of gs, ΨI and [ABA]xyl in fluctuating conditions and for several species. The validity of this model, in particular the physiological meaning of [ABA]xyl, is discussed.