In vivo sucrose stimulation of colour change in citrus fruit epicarps: Interactions between nutritional and hormonal signals

During ripening, citrus fruit-peel undergoes 'colour break', a process char acterized by the conversion of chloroplast to chromoplast. The process invo lves the progressive loss of chlorophylls and the gain of carotenoids, chan ging peel colour from green to orange. In the present work, the in vivo and in vitro effects of supplemented nutrients (sucrose and nitrogen) and phyt ohormones (gibberellins \GA\ and ethylene) on colour change in fruit epicar p of Satsuma mandarin (Citrus unshiu (Mak,) Marc,, cv, Okitsu), were studie d, The rate of colour break was correlated positively with sucrose content and negatively with nitrogen content. The removal of leaves blocked natural sucrose build-up and nitrogen reduction in the peel. Defoliation also inhi bited chlorophyll disappearance and carotenoid accumulation, thereby preven ting colour break. In vivo sucrose supplementation promoted sucrose accumul ation and advanced colour break. In both in vivo and in vitro experiments, colour change promoted by sucrose was unaffected by ethylene but delayed by GA,. in non-supplemented plants, ethylene accelerated colour break while G A, had no detectable effects. Ethylene inhibitors effectively counteracted the sucrose effects on colour change. Collectively, these results suggest t hat the chloroplast to chromoplast conversion in citrus fruit epicarps is s timulated by sucrose accumulation, The sugar regulation appears to operate via ethylene, whereas GA may act as a repressor of the sucrose-ethylene sti mulation.