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 mul/liter of ethylene, a 3-fold increase in free abscisic acid (ABA) of the flavedo could be detected after 12 hours and a 10-fold increase after 24 hours, while chlorophyll destruction did not exceed 20%. The increase in free ABA continued up to 24 hours and leveled off. Bound ABA accumulated strongly after 24 hours suggesting that excess of free ABA was being converted into the bound form. Similar increases in ABA upon exposure to ethylene occurred also 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 albedo of Eureka lemons.The synthetic cytokinin benzyladenine which retards chlorophyll breakdown delayed the ethylene-induced accumulation of free and bound ABA. GA(3) had no effect on endogenous ABA levels.Application of (14)C-2-mevalonolactone to fruits resulted in low but distinct labeling of R(F) zones corresponding to ABA markers, in both free and bound acid fractions. However, 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.