Regulation of color break in citrus fruits. Changes in pigment profiling and gene expression induced by gibberellins and nitrate, two ripening retardants

Citrus clementina fruits were repeatedly treated on-tree from mature green until breaker stages with either nitrate or gibberellin, two retardants of external ripening. The natural color break was characterized by a reduction in chlorophyll concentration, a decrease in beta, epsilon-carotenoids, beta-carotene, neoxanthin, and all-E-violaxanthin, and an increase in beta,beta-xanthophylls [mainly (9Z)-violaxanthin and,beta-cryptoxanthin]. The two retardants delayed both chlorophyll depletion and total carotenoid accumulation and in addition altered carotenoid composition. Treated fruits maintained longer the typical carotenoid composition of green fruits and reduced beta,beta-xanthophyll accumulation. Natural degreening was accompanied by a marked decrease in transcript levels of 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and geranylgeranyl reductase (CHL P) while, conversely, pheophorbide a oxygenase (PaO) and phytoene synthase (PSY) gene expression increased. Gibberellin and nitrate delayed the reduction of DXS expression and the induction of PaO and PSY transcript accumulation, while no differences in CHL P were observed. The data indicate that both ripening retardants repressed natural PaO and PSY expression, suggesting a mechanistic basis for the elevated levels of chlorophyll and lower carotenoid concentration resulting from the gibberellin and nitrogen treatments and the consequent color break delay in citrus fruit peels.