Sucrose Accumulation and Key Enzymes Activities in Different Tissues of Seed in Vegetable Soybean.

Sucrose contents and four critical enzymes activities in different seed tissues of vegetable soybean cultivars grown under field conditions were analyzed in R5.5, R6, R6.2, R6.5 and R7 stages, respectively. Sucrose contents of different seed tissues increased at early seed filling stage and then decreased at late stage; the peak sucrose level reached at the R6.2 stage. At R6.2 stage, sucrose content of three cultivars in hypocotyl were 57.6%, 53.6% and 44.2% more than those in cotyledon and 71.6%, 75.3% and 73.6% more than that in seed coat, respectively. The cotyledon contributed 90% dry weight to the total seed dry weight, thus the total seed sucrose content was primarily dependent on sucrose content in cotyledon. The highest activity of sucrose phosphate synthase (SPS) was observed in cotyledon. The cotyledon SPS activity of cv.121 was much higher than the other two cultivars. The sucrose synthase (SS) activity of different seed tissues was higher at early seed filling stage than at late filling stage. The highest SS activity was observed in hypocotyl at R5.5 stage. Greater differences were found between acid invertase (AI) and neutral invertase (NI) activities. NI activity was higher in early seed filling stage and then decreased till R7 stage. Significant differences were observed for AI activity among three seed tissues at the same seed filling stage but there were no differences for NI activity. AI activity in seed coat was much higher than that in hypocotyl and cotyledon. Negative relationship was found between AI activity and sucrose content in seed coat (r=–0.59^**). The correlation analysis between sucrose accumulation and four critical enzymes activity showed that seed sucrose content was not controlled dominantly by one enzyme. The difference of sucrose synthesis enzyme activity (SPS) minus sucrose decomposition enzyme activity (SS+AI+NI) was positively correlated with seed sucrose accumulation(r=0.53^**).