Preharvest foliar sprays of prohexadione-calcium, a gibberellin-biosynthesis inhibitor, induce chlorophyll degradation and carotenoid synthesis in Citrus rinds.

摘要

Rind colour is an important cosmetic preference for consumers when purchasing citrus fruits. As citrus fruits mature, a decrease in chlorophyll concn. unmasks the presence of carotenoid pigments; this is followed by further synthesis of carotenoids, resulting in the first appearance of the characteristic orange colour of mandarins and sweet oranges. Factors contributing to invigorating growing conditions are antagonistic to optimal rind colour development and tree vegetative vigour as well as high gibberellin and cytokinin levels are also thought to adversely affect rind colour. Thus, a method to increase preharvest rind colour by moderating vegetative vigour using a growth retardant was investigated. Prohexadione-calcium (ProCa; RegalisReg.), a gibberellin-biosynthesis inhibitor with growth retardant activity, was applied to Nules Clementine mandarin, Navelina Navel orange and Eureka lemon during the 2005 and 2006 seasons at 200 and 400 mg l-1 a.i. Rind colour rating, colorimeter measurements and pigment analyses were conducted directly after harvest, after ethylene degreening and 3 wk after cold storage. In the 2005 season, ProCa significantly increased rind colour of Nules Clementine mandarin and Navelina Navel orange directly after harvest and after ethylene degreening by decreasing chlorophyll and increasing carotenoid concn. in the flavedo of fruit, but did not affect pigment concn. of Eureka lemon despite an improvement in rind colour rating. After cold storage, however, rind colour was not significantly different among treatments. In the 2006 season, rind colour was significantly increased directly after harvest, and chlorophyll degradation together with carotenoid synthesis of all Citrus spp. tested were stimulated by the late 400 mg l-1 ProCa application. Therefore, it is suggested that foliar spray application of ProCa at a concn. of 400 mg l-1 applied 6 plus 3 wk before anticipated harvest has the potential to increase preharvest rind colour of early-maturing citrus cultivars as a result of increased carotenoid-to-chlorophyll ratio. This treatment provided a novel approach to manipulate chlorophyll degradation and carotenoid synthesis in citrus fruit, and results support the hypothesis that there may be an inverse relationship between vegetative vigour and rind colour development of citrus fruit. It is concluded that by moderating vegetative vigour through the use of growth retardants, rind colour of citrus fruit can be enhanced.