Physical Mutagen In Vitro For Genetic Resources Of Enhanced Cold Tolerance Of Watermelon (Citrullus Lanatus)

Watermelon (Citrullus Lanatus) originates from Africa, which is possessed of well character for hot and draught tolerant. However, it is severely sensitive to cold stress and is devoid of the germplasm that is cold tolerant. Watermelon can be grown almost all over the world, while its growing is largely limited seasonally. Therefore, it is urgent to enhance the genetic traits of chilling resistance in watermelon by one way of somaclonal variation combined with physical mutagen.In order to improve the present germplasm or create new germplasm, the prior researchers usually use the means of biotechnology, such as meristem culture, protoplast culture, somatic hybridization, gene transformation and somaclonal variation. Currently most of the research on somaclonal variation of watermelon is still focused on direct regeneration system. Only few papers had studied the indirect regeneration system of watermelon, meanwhile the frequency of somaclonal variation was improved greatly. Unfortunately, the variety of somaclonal variation in tissue culture is very low, so the chance of obtaining useful variation is limited. In order to increase the variety of somaclonal variation, the physical mutagen is introduced, likewise, the useful variation and mutant with excellent characteristic are selected under certain stress.In this investigation, we employed the means of somaclonal variation combined with physical mutagen to induce and select the chilling resistant candidate mutant in watermelon.Firstly, we focused on establishment of direct regeneration system of watermelon under the treatment of physical mutagen. The proper condition for cotyledon extending, adventitious shoots differentiation and half lethal dosage had been determined. And then, it was counted for the percentage of germination, the numbers of adventitious shoots and normal seedlings of watermelon. It was showed that the best concentration for inducing chilling resistant candidate mutant in 'YiXuan' was 10.9 Gy for adventitious shoots differentiation pathway, which decreased the frequency of explant regeneration to 50.0%.Also, half lethal dosage for the frequency of germination in "jingxin" and "yixuan" was statistic for seed treating.The second part of this paper was about the induction of watermelon somaclonal variation combined with physical mutagen enhancing cold tolerance by chilling treating during in vitro culture. The optimal method for inducing cold tolerant mutant was obtained by the ways of physical mutagen and chilling treatment. Significant difference was observed between -4°C and -2°C or 0°C, however, no significant difference between -2 °C and 0°C. It was showed that the effective frequency of direct adventitious shoot regeneration is 37.5% in the condition of 20Gy and -2°C treatment.Finally, the cold tolerance of candidate chilling resistant mutant was evaluated by RT-PCR. The results of cold tolerance identification through RT-PCR indicate that, plantlets regeneration after cold stress have the DREBl gene expression be enhanced intensively in plants that had endured cold treating. However, the gene in control material was not been enhanced after the same low temperature treating, or the enhancement is not visible. The candidate plants for cold tolerance need to be tested on their genetic stability.