Abstract: Preharvest seed deteriorationresistant soybean cultivar Xiangdou No. 3 and deteriorationsensitive cultivar Ningzhen No. 1 were used as experimental materials and the influences of high temperature and humidity (HTH) stress \[treatment group, (40±2) ℃/(30±2) ℃, RH (95±5)%/(70±5)%, 10 h/14 h (day/night); control group, 30±2 ℃/20±2 ℃, RH 75±5%/70±5%, 10 h/14 h (day/night)\] on seed germination, membrane lipid peroxidation, osmotic adjustment, protective enzyme activities and the expression of seed deteriorationrelated gene GmSbh1 in spring soybean seeds were studied at the physiological maturity stage for 1, 5, 10, 16, 24, 48, 96 and 168 h. HTH stress was found to cause decline of germination potential and germination percentage, especially after 24 hours stress, with the seed deteriorationresistant cultivar having a lower degree of the decline than the deteriorationsensitive one. HTH stress also led to the increase of membrane permeability and membrane lipid peroxidation, and showed more intensified peroxidation after more than 5 hours stress. The increased level of membrane lipid peroxidation in Xiangdou No. 3 was lower than that in Ningzhen No. 1 under the same stress treatment. The contents of proline and soluble sugar of the seeds significantly increased under HTH stress in both cultivars. In addition, the extent of proline content increase in Xiangdou No. 3 was significantly greater than that in Ningzhen No. 1, but the two cultivars showed no obvious difference in the increment of soluble sugar contents. The activities of protective enzymes including superoxide dismutase, peroxides and catalase increased initially then decreased in the treatment group with extended treatment time, but a slight change was observed in the control group, with the treatments presenting higher activities of the protective enzymes than the control. The amount increase of protective enzymes activities in Xiangdou No. 3 was greater than that in Ningzhen No. 1 under HTH stress compared with the control. More importantly, under HTH stress, Xiangdou No. 3 maintained highlevel enzyme activities after stress, while enzyme activities in Ningzhen No. 1 had a rapid decrease after an initial increase. Under HTH stress, expression of GmSbh1 showed an upregulated trend in Ningzhen No. 1 but a downregulated trend in Xiangdou No. 3. Hence, the main physiological and molecular mechanisms for preharvest seed deterioration of spring soybean seeds were closely related to increased membrane permeability, intensified osmo-tic adjustment, enhanced lipid peroxidation, decreased activities of protective enzymes and differential expression of GmSbh1 gene under HTH stress.

Key words: drought tolerance, physiological response., growth index, watermelon, genotype