Effects of combined high temperature and drought stress on amylose and protein contents at rice grain-filling stage

Grain-filling stage is a critical stage for rice yield and quality. There have been frequent high temperatures and droughts in several rice production regions in South China due mainly to global climate change but also to local topographic conditions. This has resulted in simultaneous decline in rice yield and quality. This study investigated the combined effects of high temperature and drought stress on rice quality at grain-filling stage via analyzing the changes in amylose and protein contents of indica rice (introgression line of H5 with induced genes of Alternanthera hiloxeroides). A pot experiment was conducted in greenhouse conditions with experimental treatments including optimal temperature plus moderate drought (OT-MD), optimal temperature plus severe drought (OT-SD), high temperature plus well-watered condition (HT-WW), high temperature plus moderate drought (HT-MD), high temperature plus severe drought (HT-SD) and then optimal temperature plus well-watered condition (OT-WW) as the control treatment (CK). In addition to CK, treatment durations were set at 10 days, 20 days, 30 days and 40 days, respectively. The results showed that while amylose content decreased, that of protein increased during high temperature period. Compared with CK, amylose content dropped by 1.783%~5.987% while that of protein increased by 1.178%~2.741% during 10~40 days heat period. A similar trend was noted for drought conditions. In other words, amylose content decreased while protein content increased during drought. Compared with CK, amylose content decreased by 1.956%~6.270% while protein content increased by 1.153%~2.944% under 10~40 days of OT-MD treatment. Stress-driven changes in amylose and protein contents were higher under OT-SD treatment than under OT-MD treatment. While combined drought and high temperature stress reduced amylose content, it enhanced protein content. The ranges for amylose and protein contents of rice due to combined high temperature and drought stress were higher than that due to the individual high temperature or drought stress. The degree of changes in amylose and protein contents increased with increasing intensity, duration and frequency of the stress. Rice quality deterioration at the 20th days of grain-filling was mainly caused by the effect of combined high temperature and drought stress. An ascending order of the stress effect arrangement on amylose and protein contents under the same treatment periods was as follows: HT-WW < OT-MD < OT-SD < HT-MD < HT-SD. This study laid the basis for drought and heat resistance of rice, a critical element in rice breeding and cultivation.