Stomatal control by fed or endogenous xylem ABA in sunflower: Interpretation of correlations between leaf water potential and stomatal conductance in anisohydric species

The stomatal conductance of several anisohydric plant species, including field-grown sunflower, frequently correlates with leaf water potential (Psi(l)), suggesting that chemical messages travelling from roots to shoots may not play an important role in stomatal control. We have performed a series of experiments in which evaporative demand, soil water status and ABA origin (endogenous or artificial) were varied in order to analyse stomatal control. Sunflower plants were subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 2.5 kPa) in the field, in the glasshouse or in a humid chamber. Sunflower plants were also fed through the xylem with varying concentrations of artificial ABA, in the glasshouse and in the field. Finally, detached leaves were fed directly with varying concentrations of ABA under three contrasting VPDs. A unique relationship between stomatal conductance (g(s)) and the concentration of ABA in the xylem sap (xylem [ABA]) was observed in all cases. In contrast, the relationship between Psi(l) and g(s) varied substantially among experiments. Its slope was positive for droughted plants and negative for ABA-fed whole plants or detached leaves, and also varied appreciably with air VPD. All observed relationships could be modelled on the basis of the assumption that Psi(l) had no controlling effect on g(s). We conclude that stomatal control depended only on the concentration of ABA in the xylem sap, and that Psi(l) was controlled by water flux through the plant (itself controlled by stomatal conductance). The possibility is also raised that differences in stomatal 'strategy' between isohydric plants (such as maize, where daytime Psi(l) does not vary appreciably with soil water status) and anisohydric plants (such as sunflower) may be accounted for by the degree of influence of Psi(l) on stomatal control, for a given level of xylem [ABA]. We propose that statistical relationships between Psi(l) and g(s) are only observed when Psi(l) has no controlling action on stomatal behaviour.