Preharvest manipulation of rind pigments of Citrus spp.

[摘要] Rind colour is one of the main cosmetic preferences consumers use when purchasing citrus(Citrus spp.) fruit. To enhance the cosmetic quality of citrus fruit, attempts were made tostimulate preharvest chlorophyll degradation and carotenoid biosynthesis to obtain a deeper,more uniform, orange rind colour in early-maturing citrus cultivars. As part of a larger studyto stimulate rind colour enhancement, an initial study was conducted on 'Eureka' lemon [C.limon (L.) Burm. f.] nursery trees to determine the concentration of various gibberellinbiosynthesis inhibitors required to obtain a biological response in citrus trees, as measured byvegetative growth. Thereafter, different concentrations of prohexadione-calcium (ProCa;Regalis®) were applied at various stages of fruit development on early-maturing citruscultivars to establish the concentration and timing of ProCa required to improve rind colourby enhancing chlorophyll degradation and carotenoid biosynthesis. In addition, a search toenhance rind colour development of early-maturing citrus cultivars was conducted byscreening various nutritional, hormonal and possible physiological stress-inducer productsand some combination treatments thereof.Multiple applications of gibberellin biosynthesis inhibitors on 'Eureka' lemon nursery treessignificantly reduced internode length and hence vegetative growth. Regalis® applied at 4 to8 g·L-1 and Sunny® (uniconazole) applied at 10 to 20 mL·L-1 had the greatest effect inreducing internode length, and were therefore identified as potential candidates for furtherfield studies to test their effect on rind colour enhancement of citrus fruit.The late, double applications (6 plus 3 weeks before anticipated harvest) of ProCa applied at400 mg·L-1 consistently improved rind colour of all Citrus spp. tested. However, these effects were more pronounced after harvest, as ethylene degreening and cold-storage stimulatedadditional chlorophyll degradation, unmasking the carotenoids, resulting in overall bettercoloured fruit. In most instances in this study, ProCa stimulated chlorophyll degradationallowing the underlying carotenoids to be expressed. Therefore, the improvement of rindcolour of citrus fruit following the application of a gibberellin biosynthesis inhibitor (400mg·L-1 ProCa applied 6 plus 3 weeks before harvest) supports the hypothesis that there maybe a relationship between vegetative vigour and rind colour development of citrus fruit.Preharvest applications of boric acid, Thiovit® (elemental sulphur), ammonium thiosulphate(ATS) and half the recommended rate of Ethrel® (48% ethephon) in combination withThiovit® and ATS stimulated chlorophyll degradation in both orange- and yellow-rindedfruit, and ColourUp® (neutralised calcium carbonate) and Figaron® (ethyclozate) stimulatedchlorophyll degradation only in orange-rinded fruit. Boric acid and the Thiovit®-ATSEthrel® combination treatment stimulated carotenoid biosynthesis in orange-rinded fruit,thereby improving the carotenoid to chlorophyll ratio. The screening of chemical productswhich stimulate chlorophyll degradation in combination with chemical products whichstimulate carotenoid biosynthesis warrants further evaluation.Worldwide, research on rind colour improvement has received attention for several decades,particularly during the 1980s. Yet, rind colour still remains a problem at the beginning ofcertain seasons. In the present study, the approach to improving rind colour was to manipulaterind pigments through the reduction of vegetative vigour, which was hypothesised to be anantagonist of chloro-chromoplast transformation. To this end, the preharvest application ofprohexadione-calcium stimulated chlorophyll degradation and carotenoid biosynthesis incitrus fruit rinds. Furthermore, preharvest applications of various chemical products provides a novel approach to stimulate chlorophyll degradation and carotenoid biosynthesis. Together,the results of this study provide potential commercial treatments that will result in deeper,more uniform orange rind colour, thereby meeting consumer needs.

[发布机构] Stellenbosch University