The metabolism of abscisic acid in relation to its reversible action on stomata in leaves of Hordeum vulgare L.

Barley leaves extracted 35 min after a 15-min pulse of 10(-6) M (14)C-labelled cis,trans-(±)-abscisic acid (ABA) showed very little breakdown of the ABA. Leaves extracted 2 h after a 30-min pulse of 10(-5) M labelled ABA showed at least 34% degradation of the hormone. The pattern of degradation was very similar for leaves kept in either light or darkness following the labelling pulse, and the products formed resembled those described in tomato fruit and bean axes. No detectable isomerization to the physiologically inactive trans,trans-(±)-ABA isomer occurred in these leaves during the 2-h period. ABA-induced stomatal closure is partially reduced 30 min after the pulse. The reversal cannot be attributed to catabolism of the hormone and must be explained by removal of the hormone into storage sites where it cannot act on the stomata.