Sugar Regulation of Plastid Interconversions in Epicarp of Citrus Fruit

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Abstract
Seasonal transformations between chloroplasts and chromoplasts, as measured by changes in chlorophyll content, in the epicarp of degreening and regreening Citrus sinensis (L.) Osbeck cv. Valencia fruit closely paralleled the accumulation and later loss of soluble sugars. At any stage of development, reversing the relative soluble sugar content in the epicarp by culturing pericarp segments on agar media with low (15 mM) or high (150 mM) sucrose concentrations reversed the direction of change in chlorophyll content. Fruit of C. madurensis Lour., which mature year around and do not regreen, also accumulated soluble sugars in the pericarp as degreening was initiated. The epicarp of C. sinensis fruit accumulated N, but total N concentrations and amino acid concentrations changed little, during degreening and regreening of C. sinensis fruit. Cessation of N fertilization reduced the tendency of pericarp segments to regreen in vitro during subsequent years, but regreening tendency was restored by inclusion of KNO3 in the media. Chloroplasts become chromoplasts and citrus fruit degreen partially in response to the accumulation of sugars in the epicarp, and the reverse transformation accompanying regreening of certain citrus species apparently occurs when accumulated sugars disappear. Change in N flux to the fruit is probably not a factor in regulating seasonal transformations, but an abundance of N in the epicarp diminishes the effects of high sugar concentrations in inducing transformation of chloroplasts to chromoplasts, thereby retarding degreening and promoting regreening.