Effects Of Low Temperature Stress At Jointing And Booting On Yield Formation In Wheat

Climate change has resulted in a continuous increase in the frequency,intensity,and duration of extreme low temperature events.Low temperature is one of the major environmental stresses,which have negative effects on crop growth and grain yield formation,leading to serious crop yield reduction.Understanding and quantifying the responding mechanism of wheat yield formation to low temperature will provide theoretical guidances for wheat production secutity and developing appropriate adaptation strategies in China.In this study,two-year environment-controlled phytotron experiments with different low temperature levels and durations at jointing and booting were conducted to investigate the impacts of low temperature with simulated daily variation on wheat yield formation,and then quantify the relationsips between low temperature and leaf photosynthesis,biomass accumulation and partitioning,grain yield and yield components.The main results were as follows:(1)The responses of gas exchange parameters(net photosynthetic rate,stomatal conductance,transpiration rate,SPAD and chlorophyll fluorescence parameters(maximum quantum yield,actual quantum yield,photochemical quenching coefficient,non-photochemical quenching coefficient)to different low temperature treatments differed between low temperature at jointing and booting.Net photsynthetic rate decreased firstly and then increased with increasing low temperature duration(from 0 to 6 days)at jointing,while decreased with increasing low temperature duration at booting.Under same low temperature duration,net photsynthetic rate decreased with decreasing temperature level(from T1 to T5)at jointing and booting.Similar responses of stomatal conductance and transpiration rate to low temperature at jointing and booting were found with net photosynthetic rate.During the treatment period,leaf SPAD was not significantly affected by low temperature.However,after low temperature treatments,leaf SPAD decreased significantly under extreme low temperature(Tmin/Tmax/Tmean of-6 ?/4?/-1 ?,T5)treatment at booting stage,while no significant difference was observed between other low temperature treatments,compared with T1(Tmin/Tmax/Tmean of 6?/16 ?/11 ?).The standardized regression coefficient showed that the major variation of net photosynthetic rate was caused by transpiration rate during low temperature treatment period,while the variation of net photosynthetic rate depended more on the changes of than on transpiration rate after low temperature treatment.Maximum quantum yield decreased firstly and then increased with increasing low temperature duration at jointing,while maximum quantum yield decreased with increasing low temperature duration at booting.With the same low temperature duration at jointing and booting,maximum quantum yield,acual quantum yield and photochemical quenching coefficient decreased with decreasing temperature level in both cultivars,while non-photochemical quenching coefficient increased with decreasing temperature level.After low temperature treatments,except for T5 treatment at booting,acual quantum yield and photochemical quenching coefficient and non-photochemical quenching coefficient in all treatments recovered rapidly to same status with T1.In addition,maximum quantum yield,acual quantum yield and photochemical quenching coefficient were significant positively correlated to net photosynthetic rate,while non-photochemical quenching coefficient was significant negatively correlated to net photosynthetic rate.Compared with maximum quantum yield and non-photochemical quenching coefficient,the correlation coefficients between acual quantum yield and photochemical quenching coefficient and net photosynthetic rate were higher.(2)Low temperature at jointing and booting reduced biomass accumulation and harvest index and influenced dry matter partitioning and translocation,leading to grain yield loss.Low temperature stress with different temperature levels and durations significantly decreased biomass accumulation at maturity and harvest index.The reduction of grain yield per plant was mainly associated with decreased biomass accumulation at maturity.For low temperatureat jointing,the reduced biomass accumulation was maily from the decreased mean leaf area index,whereas leaf nitrogen content,mean net assimilation rate and duration of photosynthesiswere relatively less affected by low temperature.After low temperature treatments at booting,compared with T1,lower leaf nitrogen content and mean leaf area index were observed in both two wheat cultivars,leading to a reduction of mean net assimilation rate,which had a significant negative influence onbiomass accumulation.In addition,there were no significant effect of low temperature at jointing on the dry matter partitioning indices for green leaves,stems and sheaths,and spikes.However,low temperature at booting decreased the dry matter partitioning indices for stems and sheaths of Yangmail6 and Xumai30,and increased the dry matter partitioning indices for green leaves before anthesis.After anthesis,the dry matter partitioning indices for stems and sheaths,and green leaves under T5 treatment at booting were significantly higher than that of Tl,while the dry matter partitioning indices for spikes were lower than that of T1.Harvest index,dry matter translocation before anthesis and dry matter accumulation after anthesis decreased significantly with the increase of accumulated cold degree days(ACDD).Compared with low temperature at jointing,harvest index,dry matter translocation before anathesis and dry matter accumulation after anathesis were more sensitive to low temperature at booting.In addition,dry matter accumulation after anathesis had a greater effect on grain yield per plant and harvest index than dry matter translocation before anathesis.Low temperature significantly decreased dry matter translocation before anathesis and dry matter translocation efficiency and the contribution of dry matter translocation before anathesis to final grain weight.Under low temperature treatments,the reduction of dry matter translocation before anathesis decreased dry matter translocation efficiency,which finally led to a lower the contribution of dry matter translocation before anathesis and a higher contribution of dry matter accumulation after anathesis to final grain weight,compared with the control.(3)Low temperature with different temperature levels and durations at jointing and booting decreased grain yield per plant,mainly by decreased spike number per plant and grain number per spike.And the impacts of low temperature on grain yield per plant were depending on cultivars and treaternent stages.Generally,grain yield per plant was more sensitive to low temperature stress at booting than at jointing,and spike number per plant and grain number per spike were more sensitive to low temperature level and duration than 1000-grain weight.For low temperatureat jointing,accumulated cold degree days(ACDD)had a greater effect on grain yield per plant and spike number per plant in Yangmai16 than Xumai30,while no obvious difference was observed between two cultivars on grain number per spike and 1000-grain weight.For low temperatureat booting,no obvious dfference was observed between two cultivars on grain yield and yield components under low temperature treatment,when considering both original and regenerated spikes.However,when only considering the original spikes,ACDD had a smaller effect on grain number per spike in Yangmail6 and Xumai30,but had a larger effect on grain yield per plant,spike number per plant and 1000-grain weight in Yangmai16 and Xumai30.The contribution of grain number per spike to the variation of wheat yield was greater than spike number per plant and 1000-grain weight at the mild low temperature level(Tmin/Tmax/Tmean of-2?/8?/3 ?,T3)in both cultivars.However,at the extreme low temperature level(T5),the major variation of grain yield per plant was caused by spike number per plant for Yangmai16 and grain number per spike for Xumai30.In general,the decreased grain yield per plant under low temperature mainly came from the decreased grain number per plant(grain number per plant = spike number per plant x grain number per plant).Thus maintaining a high grain number per plantis very important for compensating the yield losses caused by low temperature at jointing and booting.