Modeling The Impact Of Extreme Temperatures On Grain Protein And Starch In Wheat

Wheat yield and quality are being challenged by global climate change in the context of increasing food demand among the growing population.The increase in average temperatures will bring more low temperature stress(LTS)and high temperature stress(HTS),which will be the main environmental factors affecting wheat development and productivity worldwide.The LTS during early vegetative stages,particularly at jointing and booting,and HTS during post heading phase are very common in China and affect wheat grain quality.Therefore,it is imperative to quantify the impact of LTS and HTS on wheat grain quality in China.Crop simulation models have proven to be an effective tool for investigating the effects of climate change on agricultural production and estimating the effects of adaptation measures.However,poor performance and large uncertainties were found in the application of crop models under climate change,especially for extreme temperature events.Thus,improving wheat crop models under extreme temperature conditions is of utmost importance for assessing climate impacts on wheat grain quality.In this study,observed data for wheat grain quality parameters were collected from four-year environment-controlled phytotron experiments with different temperature levels and temperature durations at jointing,booting,anthesis,grain filling,and combined stress at anthesis and grain filling with two winter wheat cultivars.Model evaluation of four widely-used wheat crop models for simulating wheat grain quality under LTS and HTS were conducted,and weakness of these models for the simulations of grain protein concentration(GPC),grain yield(GY),and grain protein yield(GPY)were identified.After that,new algorithms,which quantifies the impacts of LTS and HTS effects on N dynamics,rate of grain N accumulation,and rate of grain starch accumulation were developed and integrated into WheatGrow model to improve the prediction accuracy of grain quality parameters with WheatGrow model under extreme temperature stresses.The results were of great significance for quantitative analysis of the effect of extreme temperature stresses on the grain quality and climate change impact assessments on wheat production.Firstly,four wheat models(CERES-Wheat,Nwheat,WheatGrow,and APSIM-Wheat)were evaluated using data from four-years of environment-controlled phytotron experiments to identify gaps in simulating wheat GPC and GPY for crop model improvement.Both temperature extremes increased the GPC in wheat grains but decreases the GY and GPY.For per unit increase in cold degree days(CDD,degree days below 2℃)at jointing and booting stages and in heat degree days(HDD,degree days over 30℃)at anthesis,grain filling and combined stress at anthesis and grain filling stages.GPC was increased by 0.2%to 0.4%and 1.1%to 1.6%,while GPY was decreased by 2.1%to 4.5%and 0.3%to 1.7%,respectively.The observed impact of LTS was more at booting stage than jointing stage.Whereas the effect of HTS was more at anthesis as compared with that at grain filling stage for GPC，GY,and GPY.The four models failed to simulate the effects of LTS at jointing and booting on GPC and GPY.However,Nwheat and APSIM showed a better response of GPC and GPY to HTS during grain filling than at anthesis.WheatGrow overestimated the impact of HTS on GPY,but failed to simulate the impact of HTS on GPC.CERES-Wheat requires substantial improvements under HTS at anthesis and grain filling stages for GPC,GY,and GPY.In addition,these models did not accurately simulate the responses of different cultivars to extreme temperature stresses.Above-ground N and the rate of grain N accumulation adversely affected by LTS and HTS.A new routine based on observed data from environment-controlled phytotron experiments was incorporated for quantifying extreme temperature effects on above-ground N-uptake and grain N accumulation rate in WheatGrow model.CDD and HDD were used to quantify LTS and HTS,respectively.The new extreme temperature stress algorithms introduced new parameters for quantifying the differences in the impacts of LTS and HTS at different growth stages and cultivars with different extreme temperature sensitivities.Compared with original WheatGrow model,improved WheatGrow model showed a decrease in the normalized root mean square error(NRMSE)by 79%,98%,and 88%and 69%,82%and 66%under LTS and HTS for total aboveground N,grain N accumulation and GPC,respectively.Model evaluation results indicated that the improved WheatGrow model can satisfactorily simulate the aboveground N,rate of grain N accumulation,and GPC dynamics under normal and extreme temperature scenarios.Grain starch concentration(GSC)and grain starch yield(GSY)decreased under LTS and HTS.In the case of the LTS,the rate of grain starch accumulation and GSC decreased more at booting than jointing stage.Under HTS,the rate of grain starch accumulation and GSC decreases more at anthesis as compared with those at grain filling.The observed relationships between rate of grain starch accumulation and extreme temperature stresses were integrated into the WheatGrow model to improve the GSC and GSY simulations.A new genetic parameter was also introduced in the model to show the sensitivity differences between cultivars for GSC under extreme temperature stresses.The new extreme temperature stress algorithms considered the impacts of LTS and HTS at different growth stages and between different cultivars.The improved version of WheatGrow model substantially improved the simulation accuracy of GSC and GSY,and the NRMSE decreased by 67%and 66%for LTS and 59%and 55%for HTS,respectively.Overall,the new LTS and HTS routines improved the simulated dynamics of total above-ground N,grain N accumulation,GPC,GSC,and GSY of the WheatGrow model significantly under extreme temperatures.The improvements in the WheatGrow model reduced the uncertainties in the simulations of N and starch dynamics and grain quality assessment under LTS and HTS and increased the suitability of the WheatGrow model for climate change impact of wheat grain quality.The results will provide quantitative tools and technical approaches for the analysis of the effect of extreme temperature stresses on the quality formation and the development of the adaptive measures to ensure the safety of wheat production in China under future climatic conditions,which is of great significance.