ETHYLENE-INDUCED FORMATION OF ABA IN CITRUS PEEL AS RELATED TO CHLOROPLAST TRANSFORMATIONS

When mature green harvested ''Shamouti'' oranges (Citrus sinensis L. Osbeck) were exposed to 35 .mu.l/l ethylene, a 3-fold increase in free abscisic acid (ABA) of the flavedo [the outer colored portion of citrus fruit peel] could be detected after 12 h and a 10-fold increase after 24 h, while chlorophyll destruction did not exceed 20%. The increase in free ABA continued up to 24 h and leveled off. Bound ABA accumulated strongly after 24 h, suggesting that excess of free ABA was being converted into the bound form. Similar increases in ABA upon exposure to ethylene occurred in fully mature orange fruits which had already lost all of their chlorophyll, in white and green portions of the flavedo of variegated lemons, and in the colorless of ''Eureka'' lemons. The synthetic cytokinin benzyladenine which retards chlorophyll breakdown delayed the ethylene-induced accumulation of free and bound ABA. GA3 had no effect on endogenous ABA levels. Application of 14C-2-mevalonolactone to fruits resulted in low but distinct labeling of Rf zones corresponding to ABA markers, in free and bound acid fractions. There were no significant differences in patterns of incorporation between ethylene-treated and control fruits. The results indicate that ABA accumulates in citrus peel upon exposure to ethylene irrespective of the type of plastids present in the tissue. The possible role of chloroplasts in ABA formation is discussed.