Impacts Of Solar Radiation Changes Under Global Dimming On Wheat And Rice Growth And Yield

Solar radiation is the primary energy source for crop growth and development,and plays a vital role in determining crop yield.A decadal decrease in incident global radiation,known as global dimming,has become one of the main threats with which China has to cope nowadays.Global dimming decreases the incident global radiation,but increases the fraction of diffuse radiation.Crop responses differently to diffuse radiation and direct radiation.Determining the impacts of global dimming on wheat and rice growth and yield,two main staple food crops in China,is of utmost importance for ensuring crop production in China under future changed climate.For clarifying the impacts of the combination of decreased global radiation and increased diffuse radiation fraction on wheat and rice growth and yield,this study combined statistical analysis using historical data and field experimental study.We started with statistical analysis of climate(including daily mean,maximum and minimum air temperature,precipitation,global and diffuse radiation)and crop(including phenology and yield)data in the middle and lower reaches of the Yangtze River from 1961 to 2016.We analysed the temporal changes in the global radiation and the fraction of diffuse radiation,and verified whether the global dimming impacts on crop yields can be accurately assessed by statistical models based on historical data.Secondly,field experiments with wheat(Ningmai 13)and rice(Nangeng 46)were conducted to mimic the solar radiation changes under global dimming by shading with polyethylene(PE)films in Nanjing,China,from 2013 to 2016.In each crop growing season,there were two shading treatments(T1 and T2,plots covered with different layers and thickness of films)and a control(CK,plots without cover).Based on the field experimental data,the fertilization effect of the increased diffuse radiation fraction was verified,the causes for the diffuse radiation fertilization effect were explored,and the impacts of solar radiation changes under global dimming on crop growth and yield were identified.The main research results are as follows:1.The statistical analyses using historical climate and crop data cannot reliably assess the global dimming impacts on wheat and rice yields in the middle and lower reaches of the Yangtze River.The incident global radiation reduced between 1961 and 1990,and thereafter the reduction rate slowed down or even reversed to increase,in the middle and lower reaches of the Yangtze River.Even so,the global radiation has not recovered to the level around 1961.However,the fraction of diffuse radiation in this area continuously increased during 1961-2016.Our statistical results based on historical climate and crop data demonstrated that,for wheat and rice,the decreased global radiation during 1961-2016 led to losses in yields;however,the negative correlation between rice climatic yield and diffuse radiation,and the positive correlation between wheat climatic yield and temperature were in contrast with the established understanding.These results that contrary to the established understanding were due to the significant linear correlation among statistical variables(such as between global radiation and temperature).Therefore,without the support of experimental data,statistical analysis using historical data only was insufficient to accurately evaluate the impacts of global dimming on wheat or rice yields.2.The increased diffuse radiation fraction had a fertilization effect on wheat and rice yield and aboveground biomass.The yield and aboveground biomass increased by,respectively,6.6%～7.8%and 8.2%～9.3%in wheat,and 4.2%～4.5%and 5.9%～6.2%in rice,when the diffuse radiation fraction increased 10%,relative to CK treatment.However,the diffuse radiation fertilization effect under moderate and serve air pollution mimicked in our experiments cannot compensate completely for the losses in yield and biomass caused by the decreased global radiation.Under shading treatments,the global radiation decreased by 11.8%～25.5%and the diffuse radiation fraction increased by 7.4%～20.4%in wheat growing seasons,and the global radiation decreased by 12.3%～29.9%and the diffuse radiation fraction increased by 7.9%～21.8%in rice growing seasons,and the shading treatments mimicked well the solar radiation conditions under moderate and severe air pollution.Shading treatmens decreased yield by 8.31%～16.36%and 7.81%～20.86%,respectively,and decreased aboveground biomass by 6.39%～13.45%and 5.63%～15.54%,respectively,in wheat and rice.However,the percentages of the reductions in yield and aboveground biomass were lower than that in global radiation.The yield and aboveground biomass under shading treatments can increase by,respectively,6.6%～7.8%and 8.2%～9.3%in wheat,and 4.2%～4.5%and 5.9%～6.2%in rice,when the diffuse radiation fraction increased 10%,relative to CK treatment.In addition,to keep the yield and aboveground biomass constant when the global radiation decreased 10%,the fraction of diffuse radiation that had to increase was,respectively,14.0%～14.2%and 11.5%～11.8%for wheat,and 22.1%～23.9%and 15.9%～17.7%for rice,relative to CK treatment.These results confirmed that there was a fertilization effect of the increased diffuse radiation fraction under global dimming on yield and aboveground biomass,and the fertilization effect on aboveground biomass was greater than on yield,for both wheat and rice.However,the fertilization effect of the increased diffuse radiation fraction under moderate and serve air pollution mimicked in our experiments cannot compensate completely for the losses in yield and biomass caused by the decreased global radiation.3.The diffuse radiation fertilization effect under global dimming was mainly attributed to the enhanced canopy light use efficiency but not to an improved canopy light interception;the increased canopy light use efficiency was explained not only by nonlinearities of the photosynthetic light response curves,but also by photosynthetic acclimation to global dimming.Under shading treatments,for wheat and rice,the canopy radiation use efficiency increased by 0.48%～8.14%and 1.38%～9.93%,respectively;the leaf area index increased by 0.57%～1 1.45%and 2.55%～22.91%,respectively;while the canopy light extinction coefficient decreased by 0.64%～10.60%and 2.91%～23.21%,respectively.Due to the opposite changes of leaf area index and light extinction coefficient,shading hardly altered the fraction of the incident radiation intercepted by canopy.Therefore,the diffuse radiation fertilization effect was mainly attributed to the increased canopy light use efficiency but not any improved canopy light interception.We further found that,under shading treatments,the light-saturated gross photosynthetic rate of the first and third leaves counted from the top increased by,respectively,0.57%～11.15%and 1.76%～16.18%for wheat,and 0.75%～13.08%(except shading treatments decreased by 1.71%～6.17%at stem-elongating stage in 2013)and 0.44%～7.83%(except T1 treatment decreased by 1.72%at stemelongating stage in 2015)for rice;the initial light-use efficiency of the first and third leaves increased by,respectively,1.54%～17.07%and 1.43%～15.53%for wheat,and 1.40%～13.69%and 1.30%～8.02%for rice.Shading also decreased the C/N ratio in the shaded plant,but increased green leaf nitrogen concentration by 0.94%～8.87%for wheat and 0.08%～11.27%for rice.We also found that both light-saturated gross photo synthetic rate and initial lightuse efficiency were significantly correlated with leaf nitrogen concentration.Therefore,the increased canopy light use efficiency arose not only from the saturating shape of the photosynthetic light response curves but additionally due to crop acclimation to global dimming that caused by the decreased C/N ratio in shaded plant.4.Global dimming reduced wheat and rice yield components and harvest index,especially significantly reduced the number of filled grains per ear,resulting in a smaller fertilization effect of the increased diffuse radiation fraction on yield than on aboveground biomass.For wheat,shading decreased ear number per unit area by 3.81%～9.96%,total grain number per ear by 3.44%～7.53%,filled grain number per ear by 4.13%～15.63%and thousand grain mass by 2.25%～8.96%,and ultimately decreased harvest index by 1.86%～3.66%;for rice,shading decreased ear number per unit area by 1.12%～4.50%,total grain number per ear by 4.19%～27.55%,filled grain number per ear by 5.50%～30.54%and thousand grain mass by 0.76%～8.13%,and ultimately decreased harvest index by 1.94%～6.32%.For both crops,yield loss was mainly ascribed to the reduction in the filled grain number per ear,which was predominantly caused by the decreased nitrogen uptake before heading under shading treatments.Therefore,the decreased number of filled grains per ear under shading discounted the diffuse radiation fertilization effect on wheat and rice yield,resulting in a smaller fertilization effect on yield than on aboveground biomass.In summary,there was a fertilization effect of the increased diffuse radiation under global dimming on crop productivity,due to the enhanced radiation use efficiency.However,the decreased filled grain number per ear under global dimming discounted the fertilization effect on the yield of both crops.Moreover,the enhanced canopy light use efficiency arose not only from the saturating shape of the photosynthetic light response curves but additionally due to crop acclimation to global dimming.However,the fertilization effect of the increased diffuse radiation fraction under moderate and serve air pollution mimicked in our experiments cannot compensate completely for the losses in yield and biomass caused by the decreased global radiation.The results in the present study could supply a theoretical basis and experimental data support for wheat and rice production to cope with climate change and further establishing crop models that could be used for precisely assessing the global dimming impact on crop productivity.