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To determine and elucidate the role of cool-hardening germination in the subsequent development of low-temperature tolerance in tropical and subtropical plants, 10-day-old watermelon (Citrullus lanatus) seedlings of two hybrids, Jingxin No.1 and Longfengzaochen, germinated at 18 and 30�C, respectively, were removed from growth conditions of 25�C and exposed directly to growth conditions of ?2�C for 24 h. The results showed that the seedlings germinated at 18�C, improved the subsequent seedling emergence rate at 25�C and developed tolerance to conditions of ?2�C. Low-temperature injury-associated changes of electrolyte leakage and lipid peroxidation in cotyledons and lower radicle vigour, measured as 2,3,5-triphenyltetrazolium chloride reduction, were observed in the seedlings germinated at 30�C but not in those germinated at 18�C, presumably due to the accumulation of soluble proteins. Radicle vigour showed a significantly negative correlation with cotyledon lipid peroxidation. Seed germination at less than the optimal temperature may be a feasible way to improve the cold tolerance of watermelon plants. |
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