REGULATION OF DEGREENING AND REGREENING OF CITRUS PEEL WITH SELECTED CARBOHYDRATE AND NITROGEN COMPOUNDS IN VITRO.

Abstract

The regulation of citrus fruit color by various concentrations of sugars, sugar metabolites, and nitrogen compounds was investigated in peel segments of Citrus paradisi Macf. (cv. Marsh) cultured on modified media of Murashige and Tucker. Green and yellow peel segments were cultured for degreening and regreening studies, respectively, and chlorophyll level in the individual peel segments was measured with a reflectometer. Degreening was significantly promoted by 150 mM sucrose, 300 mM glucose and fructose, or 50 mM citrate but not by 300 mM of the hexoses galactose and mannose, 300 mM of the pentoses xylose and ribose, or 25, 50, and 100 mM succinate. Regreening was significantly inhibited by 150 mM sucrose, 300 mM glucose and fructose, or 50 mM citrate and malate. Succinate and α-ketoglutarate at concentrations of 50 mM did not inhibit regreening. The inhibition of regreening by 300 mM sucrose was reduced by 33 percent with the glycolytic inhibitor iodoacetic acid at 1 mM but not by DL-glyceraldehyde at 50 mM. Neither ethanol nor potassium bicarbonate inhibited regreening, suggesting that the regulation of citrus fruit color is specific to sugars or sugar metabolites. However, pyruvate did not promote degreening or inhibit regreening because it was probably not absorbed from the media by the flavedo of the peel. These results suggest that sucrose, glucose, fructose, and citrate maintain carotenoid synthesis and accumulation in both cultured green and yellow peel segments but cause the loss of chlorophyll from green peel segments. Treatment of either the green or yellow segments with sucrose or citrate may increase the partitioning of these compounds into the mevalonic acid pathway to provide carbon for synthesis or carotenoids, resulting in degreening of the green peels and maintaining the yellow color in the yellow peels. Malonate inhibited regreening when incorporated in media at concentrations of 4 mM. This inhibition was reversed by 60 mM glutamine but not by 5 mM glutamine or KNO₃. The action of malonate on regreening may be a specific effect of malonate on plastid development rather than by modifying the partitioning of sugar metabolites into the carotenoid synthetic pathway.