As part of a larger study to improve rind color of citrus fruit, this initial study was conducted to determine the concentration of various gibberellin-biosynthesis inhibitors required to elicit a biological response in citrus trees as measured by vegetative growth. Goldschmidt (1988) hypothesized that factors contributing to invigorating growing conditions are antagonistic to optimal rind color development.
The vegetative growth of citrus trees is stimulated by various exogenous factors, viz. high temperature, high light intensity, nitrogen, and water, as well as endogenous hormones, viz. gibberellins and cytokinins. Young leaves and fruit are major sites of gibberellin biosynthesis (Salisbury and Ross, 1992; Spiegel-Roy and Goldschmidt, 1996). High endogenous gibberellin concentrations enhance stem elongation (Salisbury and Ross, 1992) and delay rind color development of citrus fruit (Garcia-Luis et al., 1985).
Growth retardants, some of which are gibberellin-biosynthesis inhibitors, reduce vegetative growth in plants by disrupting gibberellin biosynthesis (Smeirat and Qrunfleh, 1989). Aron et al. (1985) demonstrated that when paclobutrazol (Cultar®; Syngenta Crop Protection, Basel, Switzerland) was applied at 1 g·L−1 on ‘Minneola’ tangelo (Citrus reticulata × Citrus paradisi) trees just before the onset of the summer flush, it reduced shoot length, internode length, and the number of shoots developed by 41%, 76%, and 44%, respectively. Similarly, Delgado et al. (1986) showed that paclobutrazol reduced internode length, and hence shoot length, of ‘Valencia’ sweet orange (Citrus sinensis) in Cuba. Uniconazole (Sunny®; Valent BioSciences, Chicago) reduced shoot length, number of lateral shoots per terminal, number of nodes per terminal, and internode length in ‘Wichita’ pecan (Carya illinoinensis) (Graham and Storey, 2000) and ‘Cleopatra’ mandarin (C. reticulata) (Wheaton, 1989) trees. ProCa (Regalis® and Apogee®; BASF, Ludwigshafen, Germany) is used on apple (Malus ×domestica) and pear (Pyrus communis) fruit trees to reduce and control vegetative growth (Miller, 2002). Costa et al. (2001) reported that applications of 100 ppm ProCa significantly reduced shoot growth and increased fruit size in pears. ProCa acts primarily as a gibberellin-biosynthesis inhibitor, especially 3β-hydroxylation of GA20 to GA1 (Nakayama et al., 1992; Rademacher, 2001). Stover et al. (2004) found that two 500 ppm ProCa applications reduced the vegetative growth of six citrus genotypes tested by ≈40%.
In contrast to the affects of gibberellin-biosynthesis inhibitors on vegetative growth, their effects on rind color enhancement of citrus fruit are not well known. Monselise and coworkers (1976) reported that paclobutrazol contributed to the acceleration of chlorophyll degradation of sweet orange. Gilfillan and Lowe (1985) demonstrated that paclobutrazol increased ‘Satsuma’ mandarin (Citrus unshiu) rind color by 1 to 2 color rating units. This result was achieved when paclobutrazol was applied after physiological fruit drop (in November) at 1 g·L−1, as well as in summer (January and February), and suggests that paclobutrazol suppressed the early summer growth flush (November–December), which might be more important for rind color development than the late summer flush (January–February). Monselise (1986) mentioned that paclobutrazol caused a more rapid change of rind color in ‘Topaz’ tangor (C. reticulata × C. sinensis), an Israeli selection of ‘Ortanique’ tangor. Preliminary results by Barry and Van Wyk (2004) showed that when ProCa was applied 2 weeks before anticipated harvest at 100 ppm to ‘Navelina Navel’ sweet orange, rind color was improved as a result of chlorophyll degradation and carotenoid biosynthesis. No other reports on the possible affect of gibberellin-biosynthesis inhibitors on rind color enhancement of citrus fruit were found.
The principal objective of this study was to determine the concentration of various gibberellin-biosynthesis inhibitors required to retard shoot growth in citrus nursery trees. This information could then be used in field studies to test the effects of gibberellin-biosynthesis inhibitors on rind color enhancement of citrus fruit.
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