Pomegranates are presumed to have originated in the Middle East (Persia, e.g., Iran) and Southeast Asia (including Turkmenistan and Afghanistan), areas with relatively cold winters and hot summers (Castle et al., 2011). Iran is considered to be one of the major producers (Holland et al., 2009). There is considerable interest in growing pomegranate trees in tropical, subtropical, and subtemperate regions of the world, including southern Asia, Africa, and Europe, particularly, in countries such as Iran, India, Turkey, Afghanistan, Spain, Egypt, and other parts of North Africa, China, Italy, France, and the United States. Pomegranate consumption has increased recently because of studies that confirm that pomegranate may improve certain features of human health resulting from the high antioxidant content of its juice and peel and possession of properties which prevent cancer and cardiovascular diseases (Fuhrman and Aviram, 2006). Pomegranate is a natural out-crosser, the seeds germinate well and also is easily propagated by cutting (Jalikop and Kumar, 1990). It is widely distributed and have adapted to different environments. Thus, pomegranates display a wide diversity of pomological and tree characteristics. By using available germplasm, it is possible to develop cultivars with improved winterhardiness and acceptable fruit quality. Susceptibility to freezing injury is a major environmental factor limiting geographical distribution and growing season of many cultivars and affects crop quality and productivity (Thomashow, 1999). Fruit breeding efforts have recently focused on the development of cultivars with broader climatic adaptation (such as increased freezing tolerance for the northern regions), disease and pest resistance, and high fruit quality (Galletta and Ballington, 1996).
Cold hardiness is the result of complex physiological mechanisms involving many cellular and whole plant details. Moreover, winterhardiness is affected not only by tolerance to cold but also by tolerance to other factors such as frost, water logging, freeze–thaw cycles, and diseases (Steponkus, 1979). Knowledge of the hardiness of the genetic stock to winter freezing is critical to the success of tree fruit improvement programs and for selecting and breeding for improved cold hardiness. Cold hardiness evaluation can be assessed by examining cold injury after natural frost events in field trials, although there are several limitations to screening methods relying on natural frost events for cold hardiness measurement. First, field evaluation is typically established only for the short term on relatively mild, productive places which rarely receive damaging freezing. Second, the effects of infrequent frosts may be confounded with injury due to other causes. Third, the incidence, timing, and intensity of frosts are not uniform across the test place (O’Neill et al., 2001).
The evaluation of cold hardiness under controlled conditions in a freezer (artificial freezing testing) is a common test to assay plant tissue samples for cold injury (Burr et al., 1990). In this method, simple and inexpensive measurement of cold hardiness can be carried out for large number of plant samples (O’Neill et al., 2001). Ghasemi Soloklui et al. (2012) reported that EL measurement and tetrazolium stain test after controlled freezing allowed them to discriminate among pomegranate cultivars for freezing tolerance. O’Neill et al. (2001) reported that assaying for cold hardiness at the seedling stage has advantages over testing an adult tree. The seedling tests require less space and time than field assays and provide more uniform test conditions, resulting in a greater statistical precision and cost-effectiveness.
Earlier studies have shown that the genetic control of cold hardiness is very complex and mostly polygenic (Snape et al., 1997). Generation means analysis revealed that cold hardiness is a quantitative trait controlled largely by additive gene effects and, to a lesser extent, the dominance gene effects, with cold sensitivity appearing to be partially dominant (Sanghera et al., 2011). Numerous studies have shown that cold hardiness is inherited in an additive manner and progenies are intermediate between the parents, in both herbaceous and woody crops (Tang, 2002). Cold hardiness could be explained adequately by a simple additive–dominance model of gene function, although two epistatic models involving additive–additive and dominance–dominance interactions also fit the data (Arora et al., 2000).
Genetic studies on winterhardiness of pomegranate are limited. A review of the literature on temperate fruit crops shows that heritability estimates have been generated for several aspects of freezing tolerance in a few fruit crops (including plum, peach, blueberry, and apple) and the values reported are moderate to high (Arora et al., 1992; Ashworth and Wisniewski, 1991; Howe et al., 2003; Luby, 1991; Quamme, 1978). Also, genetic components of the variance have not frequently been obtained for freezing tolerance in fruit tree, but additive variance has been reported to be of major importance in apple and blueberry (Fear et al., 1985; Watkins and Spangelo, 1970). The moderate to high levels of heritability of cold hardiness in fruit tree suggest that identifying cold hardy cultivars in the germplasm and transferring that property to progenies is not difficult, but rather the combination of multiple quantitative traits including cold hardiness and fruit quality is the major challenge (Owens, 2005).
This study aimed to assess the level of genetic control of cold hardiness of the F1 progeny resulting from crosses between pomegranate cultivars with varied degrees of cold hardiness. Specific objectives include 1) estimating genetic parameters, including broad- and narrow-sense heritabilities and general and specific combining abilities, 2) prediction of breeding values and determination of potential genetic gain as well as genetic correlations between pairs of traits.
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