Elevated Temperature Effects And Its Mechanism On The Grain Yield,Nitrogen Absorption And Utilization Of Middle Rice In The Central Yangtze River Basin Of China

It is an indisputable fact that the world is growing warmer with a primary characteristic of rising air temperature,which substantially impacts sustainable rice production.In the past decades,the impacts of elevated temperature on rice yield and quality have been intensively studied using various heaters.However,there is very limited information on the effects of elevated temperature during the whole growth period on the growth and development,yield formation,and nitrogen utilization of rice crops under field conditions.In this study,an economic-sound heating device was developed and used to examine the effects of elevated temperature during the whole growth period on the growth and development,yield formation,nitrogen（N）use efficiency,and N balance of rice.Field experiments were conducted at Huazhong Agricultural University in Wuhan,Hubei Province from 2015 to 2018.Experiments were arranged in a split-plot design with warming treatment as the main plot and varieties as the subplot with four replications,and¹⁵N tracer micro-plots were set in the sub-plot.The temperature treatments were all-day warming（AW）,daytime warming（DW）,nighttime warming（NW）,and conventional control（CK）.In this study,four rice cultivars with different temperature sensitivity were tested,which were Huanghuazan（HHZ）,Shanyou63（SY63）,Yangliangyou6（YLY6）,and Liangyoupeijiu（LYPJ）.The objectives of this study were to（1）explored the effects of warming treatments on the growth and development and grain yield of rice and related mechanism,and（2）examine the responses of fertilizer N uptake,utilization,and allocation to different warming treatments of rice crop using ¹⁵N tracing method.The main results of the present study are as follows:（1）An economic-sound heating device with work efficiency of 0.165 k W h^-1 per m²was developed,which can reduce 81%operating costs compared with FATI.This device with convection heating is mainly composed of blowers,heaters,wind breaks,and a control board and is suitable for warming experiments with different temperature treatments under field conditions.This device could increase the air temperature in the rice canopy by 1.22-1.86℃in the all-day,0.63-1.18℃in the daytime,2.02-2.10℃in the nighttime,respectively,and without changing the characteristics of diurnal variation of air temperature.（2）Warming treatments had no significant effects on dry matter accumulation at panicle initiation and heading stage,but significantly decreased the dry matter accumulation after heading by 43.1%across various varieties.Consequently,the dry matter translocation efficiency of dry matter accumulation at pre-heading and contribution percentage of translocation of dry matter accumulation at pre-heading to grain was significantly lower than that of CK.Compared with CK,warming treatment significantly increased panicles m^-2 at maturity,but had no significant impacts on tillers’number at heading across different varieties.On average,AW increased panicles m^-2 by 29.4%compared with CK.In addition,warming treatments significantly decreased plant height.The plant height in AW was 14.2%lower than that of CK.Warming treatments had no significant impacts on LAI except for SY63.Averaged the years,AW increased the LAT of SY63 by 14.8%.there was no significant difference in SLW among warming treatments.（3）Elevated temperature significantly reduced rice yield and there were differences in the responses of grain yield and yield components to different warming temperatures.Compared with CK,warming treatments significantly decreased grain yield by 22.7%,61.9%,42.6%,and 47.8%for AW,5.0%,31.1%,21.7%,and 17.7%for DW,and 4.4%,30.8%,29.7%,and 22.9%for NW in 2015,2016,2017,and 2018,respectively.Warming treatments significantly reduced rice yield when the air temperature during the heading stage was higher than 29.0^oC,and the highest yield reduction of 52.3%was observed in the AW treatment.The yield reduction caused by AW,DW,and NW was mainly due to decreased seed setting rate and grain weight.Furthermore,the decrease in sink size and dry matter accumulation at the post-heading was also responsible for the yield penalty in AW and DW.The reduction of spikelets per panicle in NW was higher than in DW when warming treatments decreased spikelets per panicle.（4）Response of grain yield to different warming treatments was varied among varieties.Overaged years,warming treatments significantly decreased grain yield by 56.3%for LYPJ,45.3%for HHZ,39.7%for YLY6,and 28.4 for SY63,respectively.The high-temperature resistance presented an order of SY63>YLY6>HHZ>LYPJ.（5）Warming treatments had no significant effect on the N concentration and accumulation of different organs at pre-heading,but significantly increased N concentration of straw and grain due to the decrease in the translocation rate of N accumulation at post-heading to grain.On average,warming treatments increased the N concentration of straw and grain by 8.1%-75.0%and 2.2%-25.6%,respectively.Additionally,warming treatments significantly increased the N accumulation in straw and roots by 58.8%-118.2%and 12.3%-57.3%,respectively,but decreased grain N accumulation by 19.8%-69.3%compared with CK.The adverse effect of DW and NW treatment on grain nitrogen accumulation was weaker than that of AW treatment,but the adverse effect of NW treatment on grain nitrogen accumulation was greater than that of DW treatment.Compared with CK,NW treatment reduced grain nitrogen accumulation by 6.4%-50.3%.In addition,the nitrogen accumulation in the shoot of YLY6 and LYPJ decreased by 3.9%-11.6%,which was mainly attributed to the decrease of grain nitrogen accumulation.There was a significant difference in N use efficiency among warming treatments.Warming treatments significantly reduced NHI and NUEg,and did not significantly affect NUEb compared with CK.On average,AW exhibited 26.1%-66.8%and 28.2%-77.2%lower NHI and NUEg,respectively.（6）The results showed that the RE decreased by 5.3%-17.4%under warming treatments at maturity of rice,but the RE had no change at the heading stage under warming treatments.The decrease of RE at maturity mainly due to elevated temperature decreased absorption of panicle fertilizer by plants,and the RE of panicle fertilizer by plants under AW and CK was 70.0%,73.1%,respectively.The RE performance of SY63 and YLY6 was higher than that of HHZ and LYPJ under warming treatments.（7）Warming treatments could lead to more severe nitrogen loss after flowering.Compared with CK,nitrogen loss under AW treatment decreased by 0.65%-7.02%and0.82%-5.75%from 2016 to 2017,respectively.The increasing temperature was not conducive to the absorption of fertilizer nitrogen and soil nitrogen by HHZ,SY63,and LYPJ.Under AW treatment,the average decrease of fertilizer nitrogen was 9.8%and soil nitrogen was 11.1%.Elevated temperature significantly increased soil residual amount（RS）and residual rate（RPS）of nitrogen fertilizer,but had no significant effect on N loss（N loss）and N Loss rate（NLP）.In 2017,the average decrease of RS under AW treatment was 29.5%.To sum up,the constructed blast heating device provides an economical and feasible choice for future heating experimental research.The elevated temperature was not conducive to dry matter accumulation after flowering and significantly reduced rice yield in years with higher basal accumulated temperature,among which AW treatment had the greatest effect.The elevated temperature was not conducive to nitrogen absorption and utilization by rice,which aggravated nitrogen loss and increased nitrogen residue in the soil.Under the background of future climate warming,ensuring and improving rice seed setting rate is the key to ensuring rice yield,improving nitrogen use efficiency,and reducing fertilizer nitrogen loss.