Abstract
The hypothesis that abscisic acid (ABA) mediates stomatal response to water stress was examined in pearl millet ( Pennisetum americanum (L.) Leeke) under conditions favouring rapid water loss. Following severing of the shoot from the root system and its exposure in situ in light at low atmospheric humidity, there was a rapid reduction in leaf water potential and, following an initial transient, ‘hydropassive’ opening, a rapid reduction in stomatal aperture. While closure of stomates was virtually complete within 20 min from shoot excision, an increase in bulk leaf ABA content was not detected until 25–30 min. Thereafter, ABA content increased linearly with time at a mean rate of 3.5 ng g −1 initial fresh wt./min. The patterns, timing and extent, both of stomatal closure and ABA increase, were identical for two genotypes, BJ 104 and Serere 39. Although such results cast doubt on the involvement of ABA in the rapid response of stomata to water stress it is concluded that an internal redistribution of the hormone could easily initiate closure.
Published Version
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