Effects of temperature during grain-filling on chain length distribution and structure characteristics of 4 early-seasonindica rice cultivars were investigated under the environment-controlled conditions. The plants at flowering stage weresubjected to two temperature treatments until maturity (the mean dairy air temperature, 22 and 32C for optimum temperaturetreatment and high temperature treatment, respectively). The result showed that high temperature during grain-fillingsignificantly decreased the long B-chain content and increased the intermediate B-chain content. But the effect of hightemperature on other starch chains appeared to be cultivar-dependant. The crystalline characteristics of rice starch werealso affected by temperature during grain-filling. The intensity at 18 2q of X-ray diffraction pattern of rice samples underhigh temperature was higher than those under optimum temperature, though all rice starches performed A-crystallinetype. Moreover, the intensity at 18 2q was positive correlation with intermediate B-chain content and negative correlationwith long B-chain content.
The sucrose content, starch content and the ratio of amylose to total starch and the activities of six key enzymes, sucrose synthase(SS) , soluble starch synthesis enzyme(SSS) , ADPG pyrophosphorylase (ADPG-Ppase) , granule-boundstarch synthase (GBSS), starch branching enzyme (SBE) and debranching enzyme (DBE) , which involved in starch synthesis metabolism of developing rice grains and effect of temperature on their activities were analyzed by using two early indica varieties Zhefu49 and Jiazao935 in phyto-trons, the daily average temperature was 22 and 32℃, respectively. Results showed that the sucrose content, the starch content, and the activities of enzymes such as SS, ADPG-Ppase, SSS, SBE, and DBE were generally higher under the high temperature treatment (32℃)at the beginning stage as compared with that under low temperature treatment (22℃). However, at the middle and late stages, there were great differences in the change trend of some enzyme activities, and the association with the sucrose content and starch content was very complicated. It could be found that the activities of SS and SSS under high temperature were always lower than that under low temperature, which was closely related to the variation of the sucrose content and starch-accumulating rate. Moreover, compared with ADPG-Ppase, the SS and SSS activities were more sensitive to temperature, indicating that SS and SSS were controlled by enzyme through affecting starch accumulating rate. Difference of the amylose/total starch ratio in developing grains between 32 and 22℃was significant and the lower amylose/total starch ratio was always lower in 32℃, which was closely related to the lower activity level of GBSS. Therefore, it could be concluded that effect of temperature on amylose /total starch was more attributable to GBSS rather than to SS, SSS, ADPG-Ppase, SBE and DBE.
