HORTSCIENCE 36(5):937–940. 2001. Received for publication 30 Aug. 2000. Accepted for publication 26 Jan. 2001. Contribution from the Agricultural Research Organization, the Volcani Center, Bet Dagan, Israel. No. 420/00, 2000 series. This research was partially funded by the U.S.– Israel Binational Agricultural Research and Devel- opment (BARD) Fund No. US-2786-96R given to R.G. We thank Dr. M. Zarchi for her assistance with the statistical analysis. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper there- fore must be hereby marked advertisement solely to indicate this fact. 1 To whom reprint requests should be addressed. E-mail address: rporat@volcani.agri.gov.il Gibberellic Acid Slows Postharvest Degreening of ‘Oroblanco’ Citrus Fruits Ron Porat 1 Department of Postharvest Science of Fresh Produce, ARO, The Volcani Center, Bet Dagan 50250, Israel Xuqiao Feng, Moshe Huberman, David Galili, Raphael Goren, and Eliezer E. Goldschmidt The Kennedy-Leigh Centre for Horticultural Research, Institute for Plant Sciences and Genetics, Faculty of Agricultural, Food, and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel Additional index words. ethylene, 1-methylcyclopropene, quarantine Abstract. ‘Oroblanco’ is an early-maturing pummelo-grapefruit hybrid (Citrus grandis Osbeck x C. paradisi Macf.). The fruit are usually picked and marketed while the peel color is still green; however, in some cases they can lose this green color during postharvest shipping and storage, which diminishes their commercial value. The effects of storage temperatures, gibberellic acid (GA), ethylene, and 1-methylcyclopropene (1-MCP) on the degreening of ‘Oroblanco’ fruit were examined. Storage temperature was critical for retaining fruit color: at 2 °C the fruit remained green for a period up to 5 weeks, whereas at storage temperatures of 6, 12, and 20 °C there was a progressive increase in the rate of degreening. Applications of GA, either as preharvest sprays or as postharvest dip treatments, effectively retained the green fruit color. Ethylene exposures up to 100 μ L·L –1 for 3 days had only a slight effect on fruit degreening, and 1-MCP treatments up to 200 nL·L –1 for 16 hours had no effect at all. The slight influence of ethylene and the ineffectiveness of 1-MCP on fruit color change can not be attributed to difficulties in their application, since in the same experiments ethylene markedly induced peduncle abscis- sion, and 1-MCP effectively inhibited this ethylene effect. Accordingly, ethylene had only a relatively small effect on the induction of chlorophyllase enzyme activity in green ‘Oroblanco’ peel tissue. ‘Oroblanco’ is a triploid pummelo-grape- fruit hybrid that was generated in Riverside, Calif., by a cross between an acidless pummelo (Citrus grandis Osbeck) and a tetraploid grape- fruit (Citrus paradisi Macf.) (Soost and Cameron, 1981). The general characteristics of the fruit are similar to those of ‘Marsh’ seedless white grapefruit, despite the fact that they have a higher percentage of total soluble solids (TSS) and a lower acid content. ‘Oroblanco’ also lacks the typical bitterness of grapefruit and instead has a special, sweet aroma that is more similar to pummelo (Soost and Cameron, 1981). ‘Oroblanco’ is an early-maturing cultivar, Greenberg et al., 1992; McDonald et al., 1997). In fact, GA sprays in the orchard are currently being used as a horticultural practice to pro- long the harvest season and maintain the green color of ‘Oroblanco’ fruit (Greenberg et al., 1986). Citrus fruits are nonclimacteric, and the precise role of endogenous ethylene in the regulation of fruit maturation and peel color change is not yet clearly understood. Never- theless, exogenous ethylene is commonly be- ing used for commercial degreening of citrus fruit (Grierson et al., 1986), and it is believed that the chemical acts by increasing chloro- phyllase activity (Trebitsh et al., 1993). Stud- ies with several ethylene inhibitors, such as 2,5-norbornadiene (NBD), silver nitrate, and 1-methylcyclopropene (1-MCP), have shown that by blocking the response of the fruits to endogenous ethylene it is possible to inhibit natural degreening in ‘Shamouti’ oranges (Goldschmidt et al., 1993; Porat et al., 1999). In the current study, we have evaluated the effects on the postharvest degreening of ‘Oroblanco’ citrus fruit of different storage temperatures, preharvest and postharvest GA applications, and exposure of the fruit to vari- ous concentrations of ethylene and 1-MCP. Materials and Methods Effect of temperature and length of stor- age. ‘Oroblanco’ fruit were harvested in Octo- ber from local orchards, and their external color was evaluated immediately. Sixty fruit from each of three locations (replications) were selected, and a circle was drawn on the surface of each with a black marker. The hue angle of the peel within this circle was mea- sured with a Minolta chroma meter (model CR-200; Minolta, Ramsey, N.J.), as described elsewhere (McGuire, 1992). The hue angle of these fruit at harvest was ≈122°; a hue angle of 120° represents green color, whereas 90° rep- resents yellow. From each replicate 15 fruit were randomly selected for storage at 2, 6, 12, or 20 °C for 5 weeks at ≈90% relative humid- ity. Hue angle at the marked location was subsequently measured weekly. Effect of GA applications and 1-MCP and ethylene treatments. Prior to the October har- vest, certain trees were sprayed twice during the months of August and September with 10 mg·L –1 of GA (Machteshim, Israel) containing 0.05% Triton-B (Agan, Israel) as a surfactant. The fruit from some of these trees were also treated with GA postharvest by dipping for 20 s in 100 mg·L –1 of GA in tap water with 0.15% BB5 (CTS, Israel) as a surfactant in a combi- nation treatment. A third treatment received only the postharvest dip. Along with an un- treated control, all fruit were stored for 5 weeks at 20 °C, and hue angle was measured weekly at a marked location. Each treatment consisted of 15 fruit replicated three times. In a similar test, fruit were placed in sealed 30-L plastic tanks and treated for 16 h with 1- MCP at 20, 100, or 200 nL·L –1 or for 3 d with ethylene at concentrations of 10, 40, or 100 μ L·L –1 . In both cases the exposure to the gas was carried out at 21 °C. The concentrated which, according to its internal characteris- tics, is ready for eating in October (Soost and Cameron, 1981; Traiber et al., 1984). Com- mercially, the fruit are usually picked between October and November and are exported from Israel, mainly to Japan, when the peel is still green. Unfortunately, in some cases the fruit color changes during postharvest shipping and storage, and as a result the fruit loses some of its commercial value. Several preharvest and postharvest factors may affect the degreening of ‘Oroblanco’ fruit. Storage temperature is a major factor deter- mining the rate of postharvest deterioration of commodities (Kader, 1992). In citrus, specifi- cally, storage temperature is an important con- sideration in commercial fruit degreening (Grierson et al., 1986). Generally, increasing the temperature to 30 °C enhances chlorophyll degradation, whereas somewhat lower tem- peratures of 23 to 25 °C are optimal for caro- tenoid biosynthesis (Ahrens and Barmore, 1986; Jahn et al., 1973; Stewart and Wheaton, 1972). Thus, for ‘Oroblanco’ low storage tem- peratures should delay peel deterioration and chlorophyll loss while higher temperatures should enhance degreening. Gibberellic acid (GA) is commonly used to delay senescence and loss of chlorophyll in various citrus fruits (Coggins et al., 1992; El- Otmani et al., 1990; Garcia-Luis et al., 1992;