Rind color of citrus fruit is an important cosmetic preference of consumers when purchasing citrus fruit who generally prefer a deep orange rind color (Krajewski, 1997). As citrus fruit mature, changes in rind color are the result of decreased chlorophyll and increased carotenoid concentrations in the flavedo of the rind (Eilati et al., 1969; Goldschmidt, 1988). These changes in rind pigments are mainly the result of the senescence of chlorophyllous tissue in the flavedo and result in the transformation of chloroplasts into chromoplasts (Camara and Brangeon, 1981; El-Zeftawi and Garrett, 1978; Mayfield and Huff, 1986; Thomson, 1966). Chloro–chromoplast transformation is an important metabolic event at fruit maturity resulting in “color break” of the rind and is affected by genetic, environmental, nutritional, and hormonal factors (Goldschmidt, 1988). “Color break,” a colloquial term generally used in the citrus industries of the world, occurs when a decrease in chlorophyll concentration unmasks the presence of carotenoid pigments followed by further synthesis of carotenoids, resulting in the first appearance of the characteristic orange color of mandarins and sweet oranges (El-Zeftawi, 1978; Goldschmidt, 1988; Jackson and Davies, 1999).
Citrus rind color is primarily a genetic trait and is secondarily affected by climatic and other growing conditions that have a large influence on rind color when combined. The major factors affecting rind color are temperature (Young and Erickson, 1961), light (Sites and Reitz, 1949), nutrition (Reitz and Koo, 1960), plant water relations (Peng and Rabe, 1996), rootstock (Rabe and Von Broembsen, 1995), and phytohormones (Garcia-Luis et al., 1986). Other important factors include tree age, soil conditions, and crop load. Besides the direct effects of some of these factors on rind color, various indirect effects may also be important to rind color development. Furthermore, the interaction of seemingly minor factors may hinder rind color development.
Goldschmidt (1988) hypothesized that factors contributing to invigorating growing conditions are antagonistic to optimal rind color development. For example, young trees are more vigorous than older, mature trees (Krajewski, 1997). This vigor difference may be a major reason why fruit borne on young trees have inferior rind color compared with fruit borne on mature trees. Color development is also adversely affected by growth flushes during Stage III of fruit development caused by high fall temperatures or summer pruning. Such flushes are more common in trees bearing a low crop and in young trees of vigorous rootstock/scion combinations (Krajewski, 1997; Saunt, 2000). Rootstock vigor affects rind color development of the fruit of scions budded onto the rootstock. For example, fruit from scions budded on rough lemon (C. jambhiri Lush.) rootstock had medium–late color development, whereas ‘Troyer’ citrange [Poncirus trifoliata (L.) Raf. × C. sinensis (L.) Osbeck] rootstock resulted in 8 to 10 d earlier rind color development, and scions on ‘Swingle’ citrumelo [P. trifoliata (L.) Raf. × C. paradisi Macf.] had delayed rind color development (Rabe and Von Broembsen, 1995). Young, vigorously growing roots, beside other organs, are a major site for the biosynthesis of gibberellins and cytokinins, which are antagonistic to rind color development and are subsequently transported to the aerial portion of the tree through the xylem (Saidha et al., 1983). Vigorous rootstocks also have a higher hydraulic conductivity, allowing more water and mineral nutrients, e.g., nitrogen (N), to be transported to leaves and fruit (Syvertsen, 1981). Peng and Rabe (1996) found that when deficit irrigation caused the soil water tension to reach –70 kPa, better-colored fruit were obtained compared with normal irrigation with a soil water tension of –30 kPa. The fruit harvested was better colored as a result of lower chlorophyll levels. Koo (1988) established that excess N (greater than 160 kg·ha−1/year) increased the amount of green fruit (from 18% to 32%) when the fruit were physiologically mature and ready for harvest.
Vegetative growth of citrus trees is stimulated by various environmental factors and nutrients, namely high temperature, high light intensity, N, and water (Syvertsen, 1981), as well as endogenous hormones, namely gibberellins and cytokinins (Coggins and Lewis, 1962; Saidha et al., 1983). 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 (Mudzunga, 2000; Salisbury and Ross, 1992) and delay rind color development of citrus fruit (Garcia-Luis et al., 1985).
Goldschmidt (1988) showed that high gibberellin levels in fruit during maturation delayed chloroplast to chromoplast transformation and Gilfillan et al. (1974) showed that when GA3 was applied at color break, it resulted in unacceptably green fruit at harvest. Gibberellin-treated fruit also resulted in lower carotenoid concentration after full color development, resulting in paler colored fruit (Lewis and Coggins, 1964; Rasmussen, 1973).
Prohexadione–calcium [ProCa; BAS-125W (3-oxido-4propionyl-5-oxo-3-cyclohexene-carboxylate)] sold as Regalis® and Apogee® and developed by BASF (Limburgerhof, Germany) is used on pome fruit trees (Malus and Pyrus spp.) to reduce and control vegetative growth (Miller, 2002). Prohexadione–calcium acts primarily as a gibberellin-biosynthesis inhibitor, especially 3β-hydroxylation of GA20 to GA1 (Nakayama et al., 1992; Rademacher, 2001). Costa et al. (2001) demonstrated that repeated applications of 100 mg·L−1 ProCa significantly reduced shoot growth and increased fruit size in pears (P. communis L.).
In contrast to the affects of gibberellin-biosynthesis inhibitors on vegetative growth, their effects on rind color enhancement of fruit of Citrus spp. have not been studied extensively. Monselise et al. (1976) reported that paclobutrazol contributed to the acceleration of chlorophyll degradation of sweet orange. Gilfillan and Lowe (1985) demonstrated that paclobutrazol increased ‘Satsuma’ mandarin (C. unshiu Marc.) rind color by one to two color rating units when applied after physiological fruit drop (in November) at 1 g·L−1 as well as in summer (January and February) and suggest that paclobutrazol suppressed the early summer growth flush (November to December), which may be more important for rind color development than the late summer flush (January to February). Monselise (1986) mentioned that paclobutrazol caused a more rapid change of rind color in ‘Topaz’ tangor (C. reticulata Blanco × C. paradisi Macf.), 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 mg·L−1 to ‘Navelina Navel’ orange, rind color was improved as a result of chlorophyll degradation and carotenoid biosynthesis. No other reports in the literature on the possible affect of gibberellin-biosynthesis inhibitors on rind color enhancement of fruit of Citrus spp. were found.
The principal objective of this study was to determine whether a recently developed gibberellin-biosynthesis inhibitor would improve the rind color of citrus fruit. The concentration and timing of ProCa applications required to stimulate chlorophyll degradation and/or carotenoid synthesis were tested on several early-maturing citrus cultivars treated with different concentrations of ProCa at various stages of fruit development.
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