Effects Of Post-anthesis Heat Stress On Rice Growth,Development And Yield Formation

Extreme high temperature events are becoming more frequent in the world due to the increasing temperature and climatic variability in global.With the intensification of climate change,heat stress during reproductive stage has posed great risks to the increase of rice yield in China.Crop growth model is an effective tool to quantify the effects of climate change on crop production and to evaluate the effects of adaptation measures.However,poor predictions and systemic errors were found in the application of crop model under climate change,especially in extreme climate events.Understanding the responding mechanism of rice growth and yield formation will benefit the improvement of crop model under heat stress,and increase the simulation accuracy under climate change.Based on the observed historical heat stress,three-year experiments with different temperature levels and temperature durations during anthesis and grain filling stage were carried out in phytotron with two japonica cultivars.By analyzing of the grain growing day at different treatments,the effects of post-anthesis heat stress on development process was quantified,and the performance of the phenology module in RiceGrow model was improved under heat stress.By exploring the responses of leaf photosynthetic production,dry matter accumulation and partitioning,yield and yield component to post-anthesis heat stress,our study determinated the physiological process of yield in rice.Three heat stress indices were calculated to quantify the heat duration and intensity based on the daily maximum temperature data records observed from 228 agricultural meteorological stations in the major rice cultivation regions of South China,i.e.accumulated days of heat stress(ADHS),heat stress intensity(HSI)and heat degree-days(HDD).Results showed that large spatio-temporal variation of post-heading heat stress was found in the main rice cultivation of South China during the past 50 years.Spatially,post-heading heat stress was more severe in the central areas of South China than elsewhere,with a greater spatial difference in double-season early rice region than that in single-season rice region.The difference of phenology at each station contributed a lot to the spatial variation of heat stress.Rice suffered serious heat stress because of its exposure to the hotter part of the season during reproductive stage after heading.Temporally,post-heading heat stress generally increased in most areas of South China during the past 50 years,in particularly during the recent 15 years.The fastest increasing rate of post-anthesis heat stress was found in the southeast coastal areas and southeast plateau sub-region.The relationship between rice yield and HDD at different agricultural experimental sites showed that post-heading heat stress among different sub-region in South China explained about9-19%of the observed grain yield variability,with severe effects on double-season early rice(about 16%)than on single-season rice(about 10%).Post-anthesis heat stress accelerated the process of grain development.Significant positive correlations were observed between the shortened grain growing days and heat degree-days(HDD).With the quantification of the impact of heat stress on rice development,the response of phenology module in RiceGrow model to heat stress was improved.Performance of the improved model was tested with phenological data from the cultivars of different ecotype or maturity characteristic under post-anthesis heat stress of phytotron environments and field conditions.Validation results showed that root mean square error(RMSE),mean bias error(MBE)and variance of the distribution of differences(SI)for the prediction of grain growing days from anthesis to maturity(GDAM),decreased from 4.60d,2.65d and 7.80 in the original model to 1.58d,-0.16d and 2.40 in the improved model under post-anthesis heat stress conditions.The determination coefficient(R2)and index of agreement(IA)between simulated and observed GDAM,increased 25%and 0.21 in the improved model under post-anthesis heat stress conditions,respectively.Significant positive correlation between HDD and the prediction errors of GDAM was observed in the original RiceGrow.With an increase of 1?·d in HDD under phytotron and field conditions,the prediction error of GDAM in the original model increased 0.44 d and 0.49 d,respectively.For the improved model,the most systematic errors from the effect of post-anthesis heat stress were eliminated because there was no significant correlation between HDD and the prediction errors of GDAM.Overall,the improved model gave better predictions on grain growing days during post-anthesis stage in different rice cultivars under different heat stress environments.There was a recovery process for net photosynthetic rate(Pn)in flag leaf after a fall under heat stress during the treatment period.Under moderate high temperature conditions,leaf stomatal conductance(Cond),transpiration rate(Tr)and intercellular CO2concentration(Ci)increased with the rise of high temperature;the Pn went up again to a normal level by the adjustment of physical structure in leaves.However under extreme high temperature,the Cond,Ci and Tr in flag leaf decreased,and Pn could not completely recover.During growth stage after high temperature treatment,the Pn,LAI,SPAD and leaf area nitrogen index(LANI)all decreased with the post-anthesis developmental process.During the growth phase after heat stress during anthesis,Pn,LAI,SPAD and LANI decreased with increasing of heat stress during early grain filling stage.Nevertheless,during the later grain filling stage,the decreasing rate of each index went down.This resulted in a higher photosynthetic leaf area and photosynthetic capacity for rice plant in the late stage.As for heat stress during grain-filling stage,Pn,LAI,SPAD and LANI reduced with increasing high temperature level and duration.In this case,leaf senescence was accelerated and its physiological function weakened.Significant correlations were observed between leaf color and photosynthetic activity or leaf area.There was a linear positive relationship between SPAD and Pn,but an exponential relationship between LAI and SPAD because that a saturation phenomenon was found in the response of SPAD to LAI at the higher values.Dry matter of leaf,stem and panicle decreased under post-anthesis heat stress.Dry matter of panicle dry matter under heat stress during anthesis significantly decreased,while that of vegetative organs reduced little.The serious losses of sink capacity in panicle under heat stress during anthesis hindered the translocation of assimilates in the later stage of reproductive period,which led to the slower leaf senescence,greater stem dry matter and higher nitrogen content in vegetative organs.The effect of heat stress on sink capacity in panicle during grain-filling stage was less than that during anthesis.Heat stress during grain-filling stage accelerated the senescence of leaf and stem,which resulted in lower dry matter and nitrogen content in vegetative organs at physiological maturity.As the dynamic changes of panicle partitioning index showed that the duration of grain filling was longer for rice under heat stress during grain-filling stage than that during anthesis stage.This indicated that leaves would have a relative longer photosynthetic duration for assimilates formation.With the rise of high temperature,the ratio of soluble sugar to nitrogen in vegetative organ decreased under the treatment during anthesis,while it increased under the treatment during grain-filling stage.In addition,the contribution of apparent current photosynthesis decreased with the rise of high temperature levels and durations under post-anthesis heat stress.The contribution of pre-anthesis reserve use decreased under heat stress during anthesis,while increased with the rise of high temperature level during grain-filling stage.These results indicated that the loss of grain yield under heat stress during grain-filling stage could be partly compensated by the contribution of pre-anthesis reserve use.The aboveground dry matter,grain yield and harvest index(HI)during physiological maturity decreased with increasing high temperature level and high temperature duration.For the difference among treatments,the aboveground dry matter varied less while HI showed significant decreasing trends under heat stress during anthesis,as compared with that under heat stress during grain-filling stage.There was obvious competition for grain seeds at different positions of panicle under heat stress.The grain weight and spikelet fertility were highest at the top of panicle,followed by the middle position and the lowest at the down position.Regression analysis showed that the heat degree-days(HDD)which caused 50%reduction of spikelet fertility was 11.24?·d and 20.10?·d under heat stress during anthesis and grain-filling stages,respectively.Grain weight was hardly affected under heat stress during anthesis,while affected seriously under heat stress during grain-filling stage.With a one-unit increase of HDD,grain weight in Nanjing 41 and Wuxiangjing 14 decreased 0.49mg and 0.46mg,respectively.Generally,the effect of heat stress during anthesis on rice yield was more serious than that during grain-filling stage.Between two varieties,the higher yield in Wuxiangjingl4 under post-anthesis heat stress was attributed to the higher dry matter production and the stronger sink capacity.