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, includi ng field-grown sunflower, frequently correlates with leaf water potent ial (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 wa ter status and ABA origin (endogenous or artificial) were varied in or der to analyse stomatal control. Sunflower plants were subjected to a range of soil water potentials under contrasting air vapour pressure d eficits (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 w ith varying concentrations of artificial ABA, in the glasshouse and in the field. Finally, detached leaves were fed directly with varying co ncentrations of ABA under three contrasting VPDs. A unique relationshi p between stomatal conductance (g(s)) and the concentration of ABA in the xylem sap (xylem [ABA]) was observed in all cases. In contrast, th e relationship between Psi(l) and g(s) varied substantially among expe riments. Its slope was positive for droughted plants and negative for ABA-fed whole plants or detached leaves, and also varied appreciably w ith 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 c onclude that stomatal control depended only on the concentration of AB A in the xylem sap, and that Psi(l) was controlled by water flux throu gh the plant (itself controlled by stomatal conductance). The possibil ity is also raised that differences in stomatal 'strategy' between iso hydric plants (such as maize, where daytime Psi(l) does not vary appre ciably with soil water status) and anisohydric plants (such as sunflow er) may be accounted for by the degree of influence of Psi(l) on stoma tal control, for a given level of xylem [ABA]. We propose that statist ical relationships between Psi(l) and g(s) are only observed when Psi( l) has no controlling action on stomatal behaviour.