Germination of melon seeds at low temperatures is cultivar-dependent. By screening hundreds of accessions (Nerson and Edelstein, 1982–1984, unpublished data), it was concluded that a temperature of 14 to 15 °C can be used to distinguish between few low-temperature germinating and many nongerminating accessions. Among the low-temperature germinating accessions, three birdsnest lines from Iran germinated after 12 d of incubation at 15 °C and their germination was accelerated by several pregermination treatments (Edelstein and Kigel, 1990; Nerson et al., 1982). The threshold temperature for germination in one of these lines [‘Persia 202’ (P202)] was found to be 10 to 11 °C, but under this low temperature, germination was partial (less than 50%) and nonuniform (Edelstein and Kigel, 1990).
The ability to improve vegetable seed germination at suboptimal temperatures by infusion of phytohormones or plant growth regulators (PGRs) into the seeds was first reported by Wittwer and Bukovac (1957). Application of this idea in cucurbit seeds was reported more than 20 years later. Pregermination treatments like exogenous application of PGR or osmoconditioning are effective tools in improving melon seed germination (Nelson and Sharples, 1980; Nerson and Govers, 1986). Later studies revealed that the ability of the birdsnest line P202 to germinate at low temperature was accompanied by a relatively high endogenous gibberellin content (Edelstein et al., 1995a) and high metabolic activity (Edelstein et al., 2001).
The anatomical structure of the seedcoat acts in many species as a physical barrier to gas exchange and/or to radicle breakthrough. In Beta vulgaris (Coumans et al., 1976; Santos and Pereria, 1989) and Datura sp. (Reisman-Berman et al., 1989), the hilum was found to be the main pathway for gas exchange, and sealing the hilum prevented germination because oxygen diffusion through the seedcoat surface was not sufficient. Similar results were reported in ‘Noy Yizre'el’ (NY) melon but not in P202 (Edelstein et al., 1995b).
Genetic analysis of the relative importance of the embryo and seedcoat in low-temperature germination is complicated because the embryo is developed from both the donor and the recipient parents, whereas the seedcoat is of maternal origin only. For example, F2 and BC1 or the maternal parents and F1 seeds have different genotypic embryos but the same seedcoat genotype. Hutton and Loy (1992) did a genetic analysis of cold germinability in muskmelon by crossing a cold-germinable line to a noncold-germinable line and testing the germinability of the parents and the reciprocal F1, F2, and BC1 at 15 °C for 14 d. They concluded that three to four recessive genes and a cytoplasmatic factor controlled low-temperature germination, but they did not examine the role of the seedcoat. In contrast, other studies, using different genetic material, pointed out that dominant gene(s) are responsible for germination under low temperature (Nerson and Staub, 1989).
The main goal of the present study was to further examine the relative importance of the physical and anatomical characteristics of the seedcoat to the genotype in determining low-temperature germinability in muskmelon.
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