Effect of shading duration after heading on grain yield and quality of japonica rice in northwest China

Light intensity at grain filling stage is an important ecological factor for high rice yield and quality. To investigate the effects of shading duration on yield and quality of two high yielding (Xindao 41, Jijing 88) and two good quality (Liangjing 10, Jijing 515) Japonica rice cultivars 10 days after heading (DAH10), 20 days after heading (DAH20), heading to maturity (DAHM), and no shading as the control (CK), this experiment was conducted in Urumqi, Xinjiang, China, in 2018 and 2019. The results showed that, shading after rice heading lengthened grain filling duration, and decreased the seed setting rate, 1000-grain weight, and rice yield. The decreasing amylose content and increasing protein content under shading after heading led to an increase in chalky grain rate, degree of chalkiness, and worsened significantly paste property and taste value. Compared to CK, mean seed setting rate and thousand grain weight of DAH10, DAH20 and DAHM decreased by 3.5%, 9.7%, 11.1% and 3.7%, 7.1%, 13.1% respectively, leading to significant decrease in yield during the two years period by 11.3%, 16.5%, and 31.7%, respectively. There was no obvious influence of first ten-day shading after heading on grain amylose content and taste value. The protein content of DAH20 and DAHM in the two years increased by 20.5% and 30.8%, respectively, while amylose content decreased by 3.6% and 4.6%, respectively. This resulted in the decreasing breakdown and taste values by 15.2%, 26.1%, and 3.9%, 7.7% respectively. The effects of shading duration after heading on chalky grain rate varied with the local background light intensity. The increase of 152.1% chalky grain rate occurred in the first ten-day after heading in 2018, while the increase of 345.5% was observed 20 days after heading in 2019. High yield rice cultivars had more sensitivity to shading duration after heading compared to good quality rice cultivars. Decreases of grain filling rate and 1000-grain weight of the high yield cultivars reduced grain yield, while it led to higher chalkiness rate. In conclusion, 20 days of shading after rice heading was the key period to affect the appearance quality in rice, and the shading duration depended on the background light intensity. The longer the shading time, the worse the cooking quality of rice. This study provides a scientific basis for japonica rice production in the future climate change.