Effect And Mechanism Of Brassinolide On Germination Of Cucumber (Cummis Sativus L.) Under Cold Stress

The effect of Brassinolide (BL), the most active Brasinosteroids (BRs) treatments at low temperature on seed germination under suboptimal temperature and the physiological and biochemical changes during germination were analyzed in our study using cucumber (Cucumis sativus) variety "Xin tai mi ci" as the material. The main results are as follows:1) BL treatments at the concentration range of 10-9 to 10-7M can promote seed germination under suboptimal temperature, among which 10-8 M BL in a dark incubator at 5℃ for 3 days is most effective in improvement of germination rate, vigor index and root length during seed germination.2) BL treatment reduced relative electrical conductivity and 256nm UV absorbance of electrolyte during seed germination. The K+, Ca2+,Na+ and Mg2+ leakage in Ieachates were also reduced after 3 day BL treatment at 5℃, with the reduction of Ca2+ being the most significant. The K+, Ca2+, Na+ and Mg2+ leakage in leachates during germination changed similarly, but the ratios of K+/Na+ and Mg2+/Ca2+ by BL treatment were significantly higher than control at the peak of germination, which indicates that the ratios of K+/Na+ Mg2+/Ca2+might reflect the true level of seed vigor.3) The levels of different carbohydrates were higher in cucumber seedlings treated with BL than in control during germination, with sucrose being the most significant one, which indicated that the accumulate of carbohydrate was mainly in the form of sucrose during germination under suboptimal temperature stress. Analysis of activity of enzymes related to the metabolism of sucrose showed that BL treatment reduced the activity of sucrose synthase which contributes to the catabolism of sucrose, and increased the activity of sucrose-6-phosphate which contributes to the synthesis of sucrose. Therefore, the content of sucrose is increased by BL treatment. BL treatment increased the activity of amylase, which led to the lower level of starch in cucumber seedlings treated with BL than in control. In addition, BL treatment also increased the content of protein, free amino acid and proline in cucumber seedlings during germination.4) BL treatment increased the activity of SOD and CAT, inhibited the decrease of the activity of botanical esterase, and therefore decreased the content of MDA in cucumber seedling during germination. BL treatment also inhibited the activity of H+-ATPase, especially the one in the vacuole. The significant change of Ca2+-ATPase activity during germination suggested the possible involvement of Ca2+-ATPase in the signaling pathway regulation of germination under stress, which might provide new insight into the signal tranducation study in the future.