Abstract: High temperature or heat stress, causes thermal damage to plants, and affects plant growth, development, as well as seed longevity. By using a seed heat tolerant genotype qingyuanbendiyoucai and a seed heat susceptive genotype shaoxingaidaganyoucai, This study aimed at investigation of the effect of different temperature treaments on seed vigor and DNA methylation of these two landraces of Brassica rapa L. The result showed that the seed germination percentage and the vigor index present no significant difference from those of CK under 37°C and 4°C, the seed vigor declined significantly under 70°C of heat stress, and heat acclimation in 37°C for 2 h effectively enhanced seed thermo-tolerance. The results of MSAP analysis showed that the level of global DNA methylation decreased under 70°C of heat stress, both DNA methylation and demethylation were detected, and more DNA demethylation bands were recorded. Seed germinating potential, germination percentage, hypocotyl length, vigor index were significantly negatively correlatied with number of bands of full-methylated (both bands) at the internal cytosine, but positively correlated with the the number of bands of full-methylated (both bands) at the external cytosine. Most importantly, opposite patterns of DNA methylation were discovered in heat tolerant and susceptive seeds under 70°C heat stress, more bands of DNA demethylation were detected in the heat tolerant seeds, but more bands of DNA methylation were detected in the heat susceptive seeds, which suggested that DNA methylation and demethylation play an important role in seed heat tolerance, epigenetic regulation of gene expression by DNA methylation is important for plant to cope with heat stress.

Key words: Brassica rapa L, Heat acclimation, Heat stress, DNA methylation, Vigor index