Effects Of Heat Stress At Booting Stage On Rice Phenology And Grain Quality

Extreme heat stress events occur frequently under the context of global climate change and heat stress has become a major meteorological disaster affecting rice yield and quality.Study on the temporal and spatial changes of heat stress and its impact on rice growth,yield and quality formation is of great importance for coping with climate change and ensuring food safety.However,few studies have focused on the effects of short-term heat stress at the booting stage on rice growth,yield,and quality.A crop growth model is a powerful tool to quantitatively evaluate the impact of climate change on crop yield and quality.Due to the lack of physio-ecological processes related to extreme heat stress in most crop models,there is large uncertainty in the prediction results when crop models are applied to the simulation under extreme heat stress conditions.Hence,it is crucial to improve the prediction ability of crop growth models under heat stress.In this study,the historical meteorological data and the phenological and yield data of rice at different agricultural meteorological experimental stations were firstly analyzed to clarify the temporal and spatial distribution characteristics of pre-flowering heat stress of rice in southern China and to quantify its impact on grain yield fluctuation.On this basis,pot experiments with different heat stress treatments at the booting stage were designed and carried out to study the effects of heat stress on rice grain quality at maturity and the accumulation dynamics of grain protein and amylose.Finally,the effect of heat stress at the booting stage on the development progress of rice was quantitatively analyzed,based on which the phenological module in the Rice Grow model was improved and perfected.The results will provide a scientific basis for anti-heat stress rice cultivation techniques,breeding of high-quality varieties with heat resistance,and prediction of rice yield and quality under the context of climate change.In this study,based on the historical meteorological data of 230 meteorological stations from 1981 to 2019 and the phenological data and yield data of rice from 147 agricultural meteorological experimental stations from 1981 to 2011,various heat stress indices such as accumulated heat stress days（AHSD）,heat stress intensity（HSI）and heat degree-days（HDD）were calculated,and the spatio-temporal characteristics of pre-flowering（from panicle differentiation to flowering）heat stress from 1981 to 2019 in the rice-growing areas in southern China and its impact on grain yield was quantified.The results revealed that there were large variations in the spatial and temporal distribution of pre-flowering heat stress due to the spatial variations of rice phenology.Spatially,pre-flowering heat stress was more severe in the Middle-lower reaches of the Yangtze River plain subregion（MYS）,the Sichuan-Shanxi basin subregion（SBS）of single rice and the South of the Yangtze River hilly plain subregion（SYS）of late rice.The single rice in the Yunnan-Sichuan plateau subregion（YSS）and the early rice subregions experienced less heat stress.The spatial variation of heat stress for single rice and late rice was greater than that for early rice.Temporally,the frequency and intensity of pre-flowering heat stress in the rice-growing areas of southern China have generally increased over the past 39 years.Regions with the most obvious uptrend in heat stress were the Middle-lower reaches of the Yangtze River plain subregion（MYS）and the Sichuan-Shanxi basin subregion（SBS）for single rice and the South of the Yangtze River hilly plain subregion（SYS）for late rice.In general,the increasing trend in heat stress of single rice was the highest,followed by late rice and early rice.The increasing trend of HDD was 0.24℃·d yr^-1,0.16℃·d yr^-1 and 0.04℃·d yr^-1 for single rice,late rice and early rice,respectively.Furthermore,the effects of pre-flowering heat stress on rice grain yield during 1981-2011 were quantified based on comprehensive consideration of average temperature,heat stress and low-temperature stress before and after flowering.The results revealed that pre-flowering heat stress negatively affects grain yield in all subregions,except for the South of the Yangtze River hilly plain subregion（SYS）of late rice.The most sensitive subregions to hat stress before flowering were Middle-lower reaches of the Yangtze River plain subregion（MYS）,the Guizhou-Hunan mountain valley subregion（GHS）,and the Yunnan-Sichuan plateau subregion（YSS）.In general,a per-unit increase of HDD resulted in a 0.5%-4.9%decrease in grain yield in southern China.Random forest analysis showed that the pre-flowering heat stress was more important to rice yield than the post-flowering heat stress.Therefore,it is necessary to accelerate the research on countermeasures of pre-flowering heat stress for various regions under future climate conditions to ensure the safety of rice production.The effects of heat stress at the booting stage on grain quality at maturity of two japonica varieties with different heat-tolerance,the heat-sensitive Nanjing41 and the heat-tolerant Wuyunjing24,were investigated in phytotrons during 2014 and 2015.Rice plants were subjected to four temperature regimes(T_max/T_min/T_mean:32/22/27℃（T1）,36/26/31℃（T2）,40/30/35℃（T3）and 44/34/39℃（T4）)combined with three heat-stress durations（2（D1）,4（D2）and 6（D3）days）.The results showed that heat stress of T3 and T4 for 4 and 6 days significantly reduced panicle size,seed-setting rate and grain size;heat stress significantly accelerated the development of grains and eventually leads to reduced grain size.With the increase of temperature and the extension of heat-stress duration,the grain length,grain width,and grain thickness,chalky grain rate,brown rice rate,head rice rate,and the amylose concentration decreased,while the green rice rate and the protein concentration increased.In addition,heat-stress treatment changed the RVA profile characteristics of rice.The peak viscosity and breakdown decreased,and the pasting temperature increased.Heat degree-days（HDD）,which considered both the high-temperature level and the heat-stress duration,could better quantify the impacts of heat stress on rice grain quality at the booting stage.An increase in HDD decreased the percentage of chalky grains exponentially,and decreased amylose concentration and increased protein concentration linearly.The amylose concentration decreased by 0.085%and 0.071%,respectively,while the protein concentration increased by0.087%and 0.060%for Nanjing41 and Wuyunjing24,respectively,with every 1℃·d increase of HDD.The sensitivity of grain quality to heat stress in the two varieties differed among quality traits and with heat stress intensity.Rice-grain quality had some resistance to mild heat stress,but it could not withstand severe heat stress at booting.Short-term heat stress at the booting stage can deteriorate rice grain quality,posing a potential risk to rice quality stability.Heat stress at the booting stage significantly affected the accumulation of protein and amylose in rice grains.The results showed that approximately the first 20 days after anthesis was the main period for protein and amylose accumulation in rice grains.With the progress of grain filling,the protein concentration in grains decreased first and then increased,while the protein accumulation,amylose concentration and amylose accumulation in grains showed an"S"curve increasing trend.Moreover,grain protein concentration in grains at different spikelet positions within a panicle increased with the increase of temperature and the extension of heat-stress duration.The protein concentration in grains at the lower spikelet positions was higher than that in the grains at the upper and middle spikelet positions.While grain protein accumulation decreased with the increase of temperature and duration at T3 and T4 treatments.Furthermore,heat stress at the booting stage also significantly reduced the amylose concentration and accumulation in grains at different spikelet positions.Compared with the control,the protein accumulation and amylose accumulation at maturity decreased by 91.4%and 94.5%under T4D3,respectively.Overall,the effects of heat stress on protein and amylose accumulation in grains at the upper and middle spikelet positions were greater than that on grains at the lower spikelet position.Heat stress at the booting stage significantly reduced the distribution of nitrogen in panicle which resulted in reduced protein accumulation in grain.The results of logistic regression showed that the maximum accumulation（A）,maximum accumulation rate(R_max),and average accumulation rate(R_mean)of grain protein and amylose were negatively correlated with heat degree-days（HDD）,while the cumulative active growth period（d）showed a quadratic relationship with HDD.Grain protein concentration was negatively correlated with grain yield,while grain protein accumulation,grain amylose concentration and amylose accumulation were positively correlated with grain yield.Besides,grain protein accumulation and amylose accumulation decreased significantly with the increase of HDD.The protein and amylose accumulation in rice grains decreased by0.026 g/plant and 0.056 g/plant for each 1℃·d increase of HDD.When the HDD was about30℃·d,the accumulation of grain protein and amylose was zero,which could be considered as the critical heat-stress intensity for rice grain protein and amylose accumulation.The results can provide effective support for further improving the rice grain quality formation model under heat stress.In the process-based crop growth models,the accurate simulation of crop phenology is the basis for predicting the process of crop growth,yield,and quality formation.Heat stress can directly affect the growth rate of rice,which in turn can affect rice development progress.However,the simulation accuracy of the heat-stress effect of the growth phenological module in the existing crop growth model needs to be further improved.To evaluate the prediction ability of the phenological module in the Rice Grow model under heat stress conditions,pot experiments with different varieties,different high-temperature levels and heat-stress durations were carried out in the phytotrons during the booting stage of rice from 2012 to2017,and the experimental data acquired were used to evaluate and improve the model.The results showed moderate heat stress caused the rice flowering stage to be advanced and the growth period of rice to be shortened,while severe heat stress delayed the flowering stage and prolonged the growth period of rice.The relationship between heat stress at booting stage and the days of growth period could be described by a quadratic function.However,the original Rice Grow model failed to capture the effect of heat stress on rice phenology.Based on the existing Rice Grow model algorithms,the RTE algorithm of temperature response equation from panicle initiation to flowering in the Rice Grow model was further improved.Finally,the performance of the improved Rice Grow model was tested and evaluated by using the phenological data acquired from temperature-controlled experiments in phytotrons and field experiments of typical eco-sites under different heat stress conditions at the booting stage.The results showed the root mean square error（RMSE）decreased from 3.40 d and 6.26d to 1.86 d and 4.18 d,respectively,and the determination coefficient（R²）increased from0.89 and 0.73 to 0.97 and 0.87,respectively,for the predicted days from sowing-flowering and sowing-maturity.Overall,the prediction ability in rice phenology of the improved model for different rice varieties under different heat stress conditions at the booting stage has been greatly improved.The results can provide an effective tool for quantitative assessment of the effects of extreme heat stress on rice phenology in the context of climate change.