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The contents of Fructose (Fru), Glucose (Glu) and Sucrose (Suc), activities of sugar metabolism enzymes (NI, SAI, SPS and SS) were investigated to identify key enzymes involved in sugar accumulation process in sweet sorghum (Sorghum bicolor L. Moench) stems. We also evaluated relationships among these key factors. Ten internodes from top to base of ten sweet sorghum cultivars were used for the experiment at the physiological maturity stage (three-month-old stems). The results showed that Suc was the main sugar in all ten different cultivars, accounting for 85.2% (on average) of the total sugar content (Glu +Fru +Suc). Sweet sorghum stems of different cultivars revealed different patterns of sugar accumulation with the internodal number, increasing from top to base. There was a significant correlation between Glu and Fru contents (r2=0.96, P<0.001) in individual internodes and the correlation between Suc and hexose (Glu + Fru) content was negative (r2=0.29, P<0.001), although the quantity was not high. No relationship was
detected between Suc content and activities of NI and SS. SAI activity was positively correlated with hexose contents (R2=0.54, P<0.025) and negatively correlated with Suc content (r2=0.59, P<0.001). Although the correlation between SPS activity and Suc content was not significant (r2=0.33, P<0.1), its difference with SAI (i.e. SPS-SAI, net enzyme activity) was more related to Suc content (r2=0.68, P<0.005). These facts revealed that SAI takes a decisive role in regulating Suc accumulation in sweet sorghum stem, and SPS takes a synergy role.

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