Combined drought and heat stress affect starch synthesis and grain quality‐related traits during the grain‐filling stage of rice (Oryza sativa L.)

BACKGROUND Drought, heat and their combination negatively influence starch synthesis and its physicochemical properties at the grain‐filling stage in japonica rice; however, no reports are available on indica rice. Therefore, this present study investigated the effects of drought, heat and their combination on starch synthesis and its physicochemical properties at the grain‐filling stage of an indica rice of fundamental importance. RESULTS There was a decrease in starch content and amylose proportion relative to amylopectin, whereas an increase in soluble sugar levels under drought, heat and their combined conditions. Pasting properties, grain size, swelling power and relative crystallinity were also adversely affected, resulting in lower eating and cooking qualities. During heat and drought stress, starch crystallinity was 41.05% and 41.65% respectively, whereas under combined levels starch crystallinity was reduced significantly by 14.1%, compared to the control (42.8%). Resistant starch was reduced significantly under these stresses, directly correlated with a decline in amylose content. The major starch synthesis genes, OsAGPS2b , OsAGPLII , OsGBSSI , OsGBSSII , OsSS3A , OsPHO , OsISA1 , OsBE1 and BE2b , were down‐regulated following the onset of stress conditions in developing grains, resulting in reduced starch content. Estimated albumin content significantly decreased with the imposed stresses, whereas three other storage proteins (globulin, glutelin and prolamin) showed a mixed response. CONCLUSION Lower starch and higher soluble sugar levels impact starch–protein–lipid polymerization, resulting in reduced grain size and increased grain chalkiness. In conclusion, both drought and heat stress negatively impact rice grain quality, but their combination is more severe and synergistic.