| With the aggravation of global warming,the frequency of extreme low temperature weather increases.Huang-Huai-Hai wheat area,as one of the most important wheat planting areas in China,spring low temperature often occurs during the period before or after the jointing of wheat here,which has a great impact on wheat yield and food security.Therefore,studying the effects of low temperature stress at jointing stage on physiological metabolism of wheat under different water conditions,clarifying the physiological mechanism of irrigation before freezing to alleviate freezing injury,screening the identification indexes of freezing injury,and then establishing the quantitative evaluation model of freezing injury,which could provide theoretical basis and technical support for scientific prevention and control of low temperature freezing injury in wheat production.In this study,weak spring varieties(Yanzhan 4110 and Lankao 198)and semi-winter varieties(Zhengmai 366 and Fengdecunmai 21)were used as the research objects,irrigation(W)and non-irrigation(D)treatment were carried out one week before the occurrence of simulated freezing injury,and the potted wheat was transferred to the low temperature simulation room for low temperature simulation treatment at the pistil and stamen differentiation stage with the temperatures-2℃(T1),-4℃(T2),-4℃(T3),-8℃(T4)and-10℃(T5)and control(CK).The physiological and biochemical indexes of wheat were measured on the second day after low temperature stress,the effects of low temperature stress on physiological indexes and yield of wheat under different water conditions were analyzed,and the physiological mechanism of irrigation to mitigate the effects of low temperature stress on wheat growth and yield were identified.Based on the response changes of physiological and biochemical indexes of plants to freezing injury,a comprehensive evaluation model of freezing injury was established by using multivariate statistical analysis method,which provided technical support for scientific prevention and control of wheat freezing injury.The results are shown as follows:(1)The effects of low temperature stress on different physiological and biochemical indexes were different due to different water content,varieties and low temperature treatment levels.With the decrease of temperature,the chlorophyll index,plant water content and chlorophyll fluorescence parameters of each variety decreased,and the decrease range was different due to different water content and varieties.When the temperature dropped to T3,the decrease range differences between varieties and between water treatments were gradually obvious,the decrease range of weak spring varieties was higher than that of semi-winter varieties,but the decrease range of irrigation(W)treatment was lower than that of no irrigation(D)treatment.Under low temperature stress,the activity of antioxidant enzymes enhanced,the content of osmotic adjustment substances increased,the higher the increase range was,the stronger the cold resistance was,for different varieties and water treatments,semi-winter varieties>weak spring varieties,and irrigation treatment>no irrigation treatment.When temperature continued to drop to T4,the stress degree exceeded the plant tolerance,enzyme activity and the content of osmotic adjustment substance began to decrease,the decrease range was weak spring varieties>semi-winter varieties,no irrigation treatment>irrigation treatment,respectively.Different from the above indicators,the MDA content decreased first and then increased with the aggravation of freezing injury,in the decreasing stage,the MDA contents of D-T1 and W-T2 were the lowest,respectively,and the MDA decrease range of semi-winter varieties was higher than that of weak spring varieties;in the increasing stage,the MDA increase range of weak spring varieties were higher than that of semi-winter varieties,and the MDA increase range of non-irrigation treatment were higher than that of irrigation treatment.To sum up,irrigation treatment enhanced the antioxidant enzyme activity and the content of osmotic adjustment substance,reduced the chlorophyll degradation and the plant water loss rate,and then alleviated the damage of low temperature stress on plants.(2)The wheat yield and yield components under different water treatments and varieties decreased significantly with the aggravation of low temperature stress,the influence of low temperature treatment on grain yield between wheat varieties showed that weak spring cultivars(YZ4110 and LK198)>semi-winter cultivars(ZM366 and FDC21),while the decrease range of yield under low temperature stress between water treatments showed that no irrigation(D)treatment>irrigation(W)treatment.When the stress was serious(T3-T5),except for that some plants of D-T5 treatment froze to death and led to the reduction of regenerated panicles and lower yield compensation effect,the yield of regenerated panicles under other treatments had different compensation to the original yield,the more serious the loss of primary panicle yield,the higher the compensation effect of regenerated panicle yield on primary panicle yield.Under no irrigation,the compensation contribution rate of weak spring varieties was 12.58%-30.49%,while that of semi-winter varieties was 6.45%-24.36%.In terms of yield components,the spike number and grain number per spike decreased with the decrease of low temperature level,the decrease range of no irrigation treatment was higher than that of irrigation treatment.For 1000-grain weight,it varied with water treatment and temperature,under irrigation treatment T1-T5 and no irrigation T1-T2,the 1000-grain weight decreased with the decrease of temperature;but when temperature decreased below T3 under no irrigation,it increased.The sensitivity of low temperature for grain number per spike and grain weight at different spike positions showed that the top and lower were higher than that in the middle,the decrease ranges of grain number per spike and.grain weight at different spike positions were different due to the variety type and water content,and showed that weak spring variety>semi-winter variety,and no irrigation treatment>irrigation treatment.(3)Most of the individual physiological and biochemical indexes under different varieties,water and temperature treatments have significant correlations.In order to accurately evaluate the freezing degree of wheat,the optimal comprehensive evaluation model of freezing injury:model F(1)was constructed by screening ten key evaluation indexes of freezing injury based on the correlation and grey correlation degree method and using the principal component regression analysis;furthermore,the evaluation model of freezing injury:F(2)was constructed by transforming 19 physiological indexes into a comprehensive value based on principal component analysis and membership function method and using multiple stepwise regression analysis to screen seven key indexes.The evaluation values of each treatment were obtained by using F(1)and F(2),and was fitted with the corresponding yield loss rate(R2=0.7754,0.8505),respectively,which indicated that the regression model based on the principal component-membership function method was more accurate.In addition,seven evaluation indexes of low temperature freezing injury(chlorophyll a,leaf water content,proline,Fv/Fm,soluble protein,MDA and SOD)were determined according to the optimized regression equation;based on the constructed freezing injury comprehensive index FICI,all treatments were further divided into five categories:unfrozen(D-CK,W-CK),mild frozen(D-T1,W-T1),moderate frozen(D-T2,W-T2,W-T3),severe frozen(D-T3,WT4)and severe frozen(D-T4,W-T5,D-T5),and the corresponding yield loss rates were 0%,0-10%,10-30%,30-50%and more than 50%,respectively.In comparison,under the same temperature and moisture conditions,the freezing damage degree of weak spring varieties was more serious than that of semi-winter varieties;under the same variety and temperature conditions,the freezing damage degree of no irrigation treatment was more serious than that of irrigation treatment. |
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