Effects Of Temperature And Nitrogen On Growth And Grain Yield Of Rice And Its Physiological Mechanism

The double-season rice is one of the most important rice cropping systems in China.Maintaining double-season rice productivity in China is very important for national food security.However,the available thermal energy is insufficient for double-season rice cropping systems in the middle and lower reaches of Yangtze River.The early-season rice often experiences low temperature when it was planted early,and late-season rice often experiences low temperature when it started heading late.In rice production,farmers often attempt to advance planting in the early-season crop in order to extend the total growth duration of the two rice crops,avoid cold damage in the late-season crop,and increase annual total grain yield.In this area,low temperature stress during growing season of rice has generally declined due to global warming,but regional and periodic low temperature stress still occurs.Therefore,early planting of early-season rice will definitely increase the possibility of low temperatures experienced by the crop during the early vegetative stage.Low temperature can decrease tillering rate,inhibit leaf expansion,and reduce biomass production and grain yield.In rice production,farmers often apply large amounts of nitrogen?N?before transplanting and/or during early seedling stages to mitigate the negative effects of low temperature on rice growth and yield.However,limited knowledge is available to support this practice.To date,little attention has been paid to identify the effect of low temperature during vegetative growth stage and N on growth and yield of early-season rice.Field experiments on early-season rice were conducted under transplanting and direct seeding methods at Wuxue county,Hubei province in 2016 and 2017.Two transplanting dates?early transplatning and normal transplatning?and two N treatments?low N rate and high N rate?were designed in transplanting rice.For direct seeding rice,two sowing dates?early sowing and normal sowing?and two N treatments?low N rate and high N rate?were chose.The objectives of this study were to 1)determine whether the negative effects of low temperature on early-season rice grain yield can be mitigated by applying more basal N fertilizer,and 2)to identify the underlying agronomical and physiological mechanisms of high basal N application for alleviating the effects of low temperature.In addition,a pot experiment was conducted with two temperature treatments?normal and low temperature?and two N treatments?low N rate and high N rate?in 2017 in Huazhong Agricultural University aiming to identify the effects of temperature and N on rice photosynthesis.The main results are as follows:?1?Compared with normal transplanting and normal sowing,early transplanting and early sowing decreased the growth temperature of rice,respectively,especially during early growth stage.The effects of low temperature on grain yield were different under different N treatments.At the low N rate,the low temperature associated with early transplanting reduced the grain yield of early-season rice by 20.1%in 2016 and by 7.0%in 2017,and the low temperature associated with early sowing decreased the grain yield of early-season rice by 13.2%in 2016 and by 5.5%in 2017.However,transplanting date and sowing date did not significantly affect grain yield at the high N rate.The reductions in grain yield due to early transplanting and early sowing in the low N rate were explained by spikelets m^-2 rather than grain filling percentage and grain weight.From another perspective,these yield reductions were mainly attributed to decreased total dry weight rather than harvest index.At the low N rate,the stem number,leaf area index,N uptake,and dry weight were decreased by early transplanting and early sowing during the early growth stage,which was contributed to the low total dry weight.?2?The effects of N on grain yield were different depending on transplanting date and sowing date.For transplanting rice,the high N rate increased the grain yield by 21.7%in 2016 and by 22.1%in 2017 under early transplanting,but it only increased the grain yield by 1.9%in 2016 and by 10.2%in 2017 under normal transplanting.For direct seeding rice,the high N rate increased the grain yield by 11.8%in 2016 and by 11.7%in2017 under early sowing,but it did not significantly affect grain yield under normal sowing.The increase in grain yield due to increased N rate under early transplanting and under early sowing were explained by spikelets m^-2 and total dry weight.The high N rate increased stem number,leaf area index and N uptake,which was contributed to significant increase in total dry weight under early transplanting and early sowing.?3?Variations in N supply and temperature can both have dramatic effects on leaf photosynthetic rate?A?,which was significantly higher at high N rate than at low N rate under both normal and low temperature.A significantly decreased at low temperature,but the decrease was less at the high N rate?32.5%?than at low N rate?47.4%?.This suggested that high N can alleviate the inhibition of photosynthesis under low temperature stress.Under short-term low temperature,the decreases in maximum electron transport rate(J_max)and mesophyll conductance?g_m?at high N rate were 28.5%and 55.1%,respectively;they were decreased by 47.6%and 66.5%,respectively,at low N rate.The relatively less decreases in J_max and g_m at high N rate accounted for its less depression in A under low temperature.A and J_max were gradually decreased with the increasing duration of low temperature stress,but the depression in A and J_max was always lower for high N rate than for low N rate.In conclusion,the negative effects of low temperature on growth and grain yield of early-season rice can be mitigated by applying more basal N fertilizer under both transplanting and direct seeding methods.The main reasons for high N rate to mitigate the negative effects of low temperature are as follows:the high N rate promoted N uptake,tillering,and leaf area development under low temperature,thereby increasing the total dry weight and spikelets m^-2,and ultimately increasing grain yield.However,a large amount of N application,especially the increase of basal N,not only raises production costs,decreases N use efficiency but also imposes serious environmental costs.Therefore,increasing basal N fertilizer is not a suitable way to mitigate the negative effects of low temperature on growth and grain yield of early-season rice.