Stomatal responsiveness to leaf water status in common bean (Phaseolus vulgaris L.) is a function of time of day

Stomatal response to leaf water status was experimentally manipulated by pressurizing the soil and roots of potted common bean plants enclosed in a custom-built root pressure chamber. Gas exchange was monitored using a whole-plant cuvette and plant water status using in situ leaf psychrometry. Bean plants re-opened their stomata upon pressurization, but the extent of re-opening was strongly dependent on the time of day when the soil was pressurized, with maximum re-opening in the morning hours and limited re-opening in the afternoon. Neither;leaf nor xylem abscisic acid concentrations could explain the reduced response to pressurization in the afternoon. The significance of this phenomenon is discussed in the context of circadian rhythms and of other recent findings on the 'apparent feed-forward response' of the stomata of some species to vapour pressure deficit.