CHANGES IN CAROTENOID COMPOSITION IN SENESCING LEAVES OF HORDEUM-VULGARE L CV DYAN

Changes in carotenoid composition were monitored during senescence of the primary leaf of seedlings of Hordeum vulgare L. cv. Dyan. Senescen ce occurred due to the normal course of leaf development or was induce d by incubation of attached leaves in darkness, or by incubation of de tached leaf segments in either continuous light or darkness. Carotenoi d and abscisic acid levels were monitored in the same tissue extracts, prepared from either attached leaves or detached leaf segments, throu ghout the course of foliar senescence. The results obtained show that the increase in endogenous levels of abscisic acid, induced by senesce nce, correlated with enhanced epoxidation of the xanthophyll cycle, i. e., increased conversion of zeaxanthin to antheraxanthin and all-trans -violaxanthin. Additionally, an increase in abscisic acid levels occur red concomitant with a decrease in all-trans-violaxanthin and 9'-cis-n eoaxanthin, suggesting an apparent 1:1 relationship on a molar basis. It is therefore proposed that enhanced abscisic acid production, due t o foliar senescence, arises from fluctuations in carotenoid turnover. Evidence is also presented to suggest that abscisic acid production an d xanthophyll cycle turnover are not independent reactions in the proc ess of leaf senescence.