Fruit Age and Growth Regulator Effects on the Quantity and Structure of the Epicuticular Wax of ‘Washington’ Navel Orange Fruit

Abstract

Changes in ‘Washington’ navel orange [ Citrus sinensis (L.). Osbeck] fruit epicuticular wax quantity and structure were followed during fruit development, maturation, and senescence. Whole-tree sprays of 10 ppm GA 3 , 16 ppm 2,4-D, and a combination of GA 3 (10 ppm) plus 2,4-D (16 ppm) were made at fruit color-break, and effects on wax were examined at 4-week intervals. The total epicuticular wax produced per fruit and per unit of fruit surface area increased with time on control and on treated fruit. GA 3 -treated fruit produced significantly lower amounts of wax per unit of surface area than did control or fruit treated with 2,4-D alone. Total epicuticular wax per unit of fruit surface area of the control was similar to that of the 2,4-D-treated fruit until late in the season, when that of the control significantly exceeded that of the 2,4-D treatment. Wax per unit of fruit area of the GA 3 -treated fruit was not significantly different from that of the GA 3 plus 2,4-D-treated fruit until late in the season, when GA 3 plus 2,4-D treatment resulted in significantly higher values. Scanning electron microscopy revealed significant changes in surface morphology and ultrastructure of the epicuticular wax during fruit development, maturation, and senescence. Control fruit and 2,4-D-treated fruit were similar, but GA 3 delayed ultrastructural changes that were seen in the control fruit. The incidence of ‘sticky’ rinds was reduced significantly by GA 3 treatment, and ‘sticky’ rind surface wax had a structure similar to that of senescing surfaces. Chemical names used: (2,4-dichlorophenoxy)acetic acid (2,4-D); gibberellic acid (GA 3 ).