Starch accumulation in rice grains subjected to drought during grain filling stage

Drought stress during the grain filling stage severely affects the quality and quantity of starch in rice grains. The enzymes such as ADP-glucose pyrophosphorylase (AGPase, EC 2.7.7.27) and starch synthase (SS, EC 2.4.1.21) are the key regulatory enzymes involved in the starch biosynthesis. In this study, the activity of the AGPase and starch synthase (SS) was correlated with the qualitative and quantitative parameters such as sucrose, starch, amylose, amylopectin, and resistant starch in leaves, roots, and grains of drought tolerant (N22) and drought susceptible (IR64) cultivars under applied water deficit stress (WDS). Drought stress enhanced the remobilization of stored starch from leaves to developing rice grains which was positively correlated with a decrease in the starch and starch synthase activity in leaves. Starch accumulation in developing grains was positively correlated with an increase in the AGPase and SS activity under drought. It was found that starch, amylopectin, and sucrose content in developing grains increased under water deficit stress (WDS), while amylose content decreased in both the varieties. However, in leaves, the SS activity decreased while AGPase activity was found to be increased under WDS in both varieties. Decreased starch content in matured grains was due to shortening of grain filling stage as drought stress caused early plant senescence. Yield reduction under drought was more in susceptible variety IR64 as compared to tolerant genotype N22.