Citrus is the most economically important fruit tree crop in Florida with an estimated economic value of $1.35 billion annually (National Agricultural Statistics Services, 2012). Low temperatures are among the greatest risk factors for U.S. citrus production. Most commercial citrus cultivars are cold-sensitive (Soost and Roose, 1996) and damage will often result when exposed to temperatures below –2.2 °C (Yelenosky, 1985). In the 1980s there were multiple severe freeze events in Florida (1981, 1982, 1983, 1985, and 1989) that resulted in crop and yield losses in excess of $1 billion U.S. (Tignor et al., 1998). After these extreme freezes, citrus production shifted to the warmer southern parts of the state (Miller, 1991; Yelenosky, 1996). Citrus growers take preventative measures to minimize crop losses from freezes such as use of trunk wraps and water applied through elevated microsprinklers (Bourgeois et al., 1990; Davies et al., 1984; Jackson et al., 1983). However, the effectiveness of these techniques is dependent on wind conditions, types of freezes (advective vs. radiation), water availability, and tree and grove size (Bourgeois et al., 1990; Ebel et al., 2005). Protection using microsprinklers is compromised by high wind speeds (Nisbitt et al., 2000). Developing more cold-tolerant citrus varieties through breeding and selection has long been considered the most effective long-term solution (Grosser et al., 2000; Yelenosky, 1985).
Citrus and Citrus relatives are members of the family Rutaceae. The subtribe Citrinae is composed of Citrus (mandarins, oranges, pummelos, grapefruits, papedas, limes, lemons, citrons, and sour oranges); Poncirus (deciduous trifoliate oranges); Fortunella (kumquats); Microcitrus and Eremocitrus (both Australian natives); and Clymenia (Penjor et al., 2013). There is considerable morphological and ecological variation within this group. With Citrus, cold-hardiness ranges from cold-tolerant to cold-sensitive (Soost and Roose, 1996). Poncirus and Fortunella are considered the most cold-tolerant genera that are cross-compatible with Citrus. Poncirus trifoliata reportedly can withstand temperatures as low as –30 °C with proper acclimation (Lang et al., 2005) and is used as a cold-tolerant rootstock for some commercial citrus production (Ebel et al., 2008; Tignor et al., 1998). The most cold-tolerant commercial Citrus variety is considered C. unshiu (Satsuma mandarin), whereas C. aurantifolia Swing. (Mexican Lime), C. limon L. Burm. f. (Lemon), and C. medica L. (Citron) are considered the most susceptible to freeze damage (Davies and Albrigo, 1994; Grosser et al., 1998).
A period of acclimation is important for inducing cold tolerance in Citrus. Cold tolerance in many plant species is not constitutively expressed; it is induced in response to reduction in daylength and the exposure to non-freezing chilling temperatures. Periods of warm weather before a freeze event will make citrus plants less cold-acclimated and thus more susceptible to the freezing conditions (Davies and Albrigo, 1994). Gene expression differences during acclimation have been associated with freeze tolerance in Citrus (He et al., 2012; Lang et al., 2005; Zhang et al., 2005). Genes for cold tolerance have been identified in P. trifoliata, C. grandis (= C. maxima), C. paradisi, C. sinensis, and C. jambhiri (Champ et al., 2007; He et al., 2012; Long et al., 2012; Sahin-Çevik and Moore, 2006; Webber et al., 2003). Use of cold-tolerant rootstocks such as Poncirus can increase the expression of cold tolerance associated genes in cold-tolerant scions (Ebel et al., 2005; Huang et al., 2011; Yelenosky and Vu; 1992).
Non-cultivated Citrus and Citrus relatives have economically important traits that could be extremely valuable in the development of new cultivars (Grosser and Gmitter, 1990). Breeding within the Citrus genepool is complicated by many factors including prolonged juvenility, self- and cross-incompatibility, polyembryony (apomixis), heterozygosity, and inbreeding depression (Grosser and Gmitter, 1990; Soost and Cameron, 1975; Soost and Roose, 1996). However, genetic transformation techniques such as protoplast transformation (Fleming et al., 2000; Grosser and Gmitter, 1990), particle bombardment (Yao et al., 1996), and Agrobacterium-mediated transformation (Luth and Moore, 1999; Moore et al., 1992) may be used to develop genotypes with cold resistance genes from distant relatives but with desired fruit quality.
The objective of this study was to survey genotypes within the Rutaceae for freeze damage as an indication of cold tolerance. Seedlings from 92 highly diverse seed-source genotypes were exposed to natural freeze events in east–central Florida. Defoliation and dieback were assessed in the winters of 2010 and 2011.
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