Spraying a solution of abscisic acid (ABA) of O· 5 or 5· 0 ,..g/ml onto intact barley plants led to an accumulation of free proline in the leaves in certain experiments, as did suspension of the root system in an ABA solution. Proline did not accumulate in the roots of intact barley so treated even when the hormone was taken into the plant through the roots.
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Aspinall et al. (1973) studied this question.