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. Senescence occurred due to the normal course of leaf development or was induced by incubation of attached leaves in darkness, or by incubation of detached leaf segments in either continuous light or darkness. Carotenoid and abscisic acid levels were monitored in the same tissue extracts, prepared from either attached leaves or detached leaf segments, throughout the course of foliar senescence. The results obtained show that the increase in endogenous levels of abscisic acid, induced by senescence, 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 occurred concomitant with a decrease in all-trans-violaxanthin and 9'-cis-neoaxanthin, suggesting an apparent 1:1 relationship on a molar basis. It is therefore proposed that enhanced abscisic acid production, due to foliar senescence, arises from fluctuations in carotenoid turnover. Evidence is also presented to suggest that abscisic acid production and xanthophyll cycle turnover are not independent reactions in the process of leaf senescence.