Fruit Age and Growth Regulator Effects on the Quantity and Structure of the Epicuticular Wax of ‘Washington’ Navel Orange Fruit Mohamed El-Otmani 1 and Charles W. Coggins, Jr .2 Department of Botany and Plant Sciences, University of California, Riverside, CA 92521-0124 Additional index words. Citrus sinensis, gibberellic acid, (2,4-dichlorophenoxy)acetic acid, ‘sticky,’ rinds, senescence, ultrastructure, scanning electron microscopy 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 GA3, 16 ppm 2,4-D, and a combination of GA3 (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. GA3-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 GA3-treated fruit was not significantly different from that of the GA3 plus 2,4-D-treated fruit until late in the season, when GA3 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 GA3 delayed ultrastructural changes that were seen in the control fruit. The incidence of ‘sticky’ rinds was reduced significantly by GA3 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 (GA3). J. A mer . Soc . Hort . Sci . 110(3):371-378. 1985. The citrus rind softens during, and subsequent to, fruit mat- uration. This softening leads to a number of rind disorders which reduce preharvest life, shelf life, and market value of fruit. When applied preharvest to citrus trees, GA3 has been shown to reduce rind staining (7), increase protection against water spot (16), decrease the rate of peel softening (8 ), reduce the incidence of puffy rinds (15), delay the development of ‘sticky’ rinds under preharvest as well as postharvest conditions, and reduce rind susceptibility to decay caused by microorganisms, such as Penicillium (6 ). These benefits have resulted in the widespread use of GA3 to maintain citrus fruit peel at a physi- ologically young stage for a period of 6 to 8 months. Fruit drop is another problem when the harvest season is long. It was shown that GA3, when used alone, had no (4) or little (9) effect on abscission of grapefruit. On the other hand, 2,4- D significantly reduced grapefruit preharvest drop when used alone (9, 18) or in combination with GA3 (4, 9). The 2,4-D plus GA3 combination was shown to reduce fruit drop and im- prove rind firmness (9). Plant cuticles and surface waxes are very important in regu- lating water loss, uptake and loss of chemicals, and they often play a role in reducing fungal invasion and mechanical abrasion, in resisting pollutants, and in resisting freezing. Some research on citrus epicuticular wax has been done. Published studies have concentrated on the three stages of fruit development, as defined by Bain (5) and Holtzhausen (14), where Stage I = cell divi- sion, Stage II = cell enlargement, and Stage III = maturation. Very limited information is available regarding early stages of senescence (3, 17), and no published information was found Received for publication 4 Sept. 1984. This research was conducted in partial fulfillment of the requirement of the PhD degree by the senior author. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked adver- tisement solely to indicate this fact. R esent address: Complexe Horticole d’Agadir, B.P.438, Agadir, Morocco. 2To whom correspondence should be addressed. regarding later stages of senescence. Previous studies have re- lated fruit age with quantity of wax (2, 3, 11, 17), wax com- position (11), and surface morphology changes (2, 3, 11). Possible control exerted by growth regulators on these parameters has not yet been investigated. The objectives of this study were 1) to examine the fruit epicuticular wax quantitative and ultrastructural changes throughout fruit growth, development, maturation, and senes- cence, including the accumulation of ‘sticky’ rind exudates dur- ing senescence; and 2) to investigate the effect of GA3 and 2,4- D on these parameters as related to peel aging and quality in ‘Washington’ navel orange. Materials and Methods Plant Material Ninety-six 14-year-old ‘Washington’ navel orange trees [Cit- rus sinensis (L.) Osbeck] on ‘Troyer’ citrange rootstock [Citrus sinensis (L.) Osbeck X Poncirus trifoliata (L.) Raf.], planted in a gravelly, well-drained, sandy loam soil with a spacing of 5.5 m X 6.0 m and with under-tree sprinklers, were the source of all fruit material used in this study. The grove was located near Riverside, Calif, and data were collected from 25 May 1983 through 19 Apr. 1984. Treatment application and experimental design GA3 at a rate of 10 ppm, and the isopropyl ester of 2,4-D at a concentration of 16 ppm acid equivalent were applied sepa- rately and in combination to whole trees at fruit color-break on 6 Oct. 1983; control trees were untreated. A nonionic wetting agent containing alkylaryl polyoxyethylene glycols (60%) as active ingredient (Western Farm Service Spreader) was used at a rate of 12 ml/100 liters solution. Treatments were applied to the point of runoff (40 liters/tree). A randomized complete block design with six 4-tree replications per treatment was used. J. Amer. Soc. Hort. Sci. 110(3):371-378. 1985. 371