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Experimental results are presented which show that abscisic acid (ABA) causes stomatal closure only if the stomatal complex is adjacent to live epidermal cells. It is further shown that ABA acts by affecting solute fluxes into and out of epidermal and guard cells. Live epidermal cells function as recipients for solutes and thereby assist their movement out of the guard cells. ABA-mediated solute leakage from guard cells alone does not suffice to cause stomatal closure within one hour.
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Itai et al. (1978) studied this question.
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