Physiological Traits Of Drought Resistance And Mechanism Of Resistance To Drought Improved By Nitrogen Application Of Maize

Drought is one of the most major envirnmental stresses influencing grain yield. It is a key and efficiencial biological way to solve the water stress to breed new maize hybrids with both stronger drought tolerance. With three experiments cultivated in pots and in fields, we studied systematically the expression of these characters such as growth and development, morphology, yield and physiological biochemistry of the different genotype maize under water stress though the methods of blurring subordination function, grey correlation degree, analysis of principal component. Appraised the drought resistance of different maize variety based on drought index（DI） of yield, analyzed the correlations between evaluation parameters and tested values of traits and evaluate N application rates management effects on physiological characteristics and grain yield of maize at the seeding stage under water stress. It got the following main conclusions:1. Comparing with contrast, the biomass and yield of maize varieties under water stress decreased obviously, and ASI added, leaves area reduced, height of plant and ear position declined, stalk became thinner, ears were shorter and smaller, barren ear tip increased more, number of grain per ear reduced less and grain weight reduced lighter, which leaded to yield decline. ASI and stay green were relatate to drought tolerance index. With comprehensive appraisement and cluster analysis by different parameters and indexes, the droughr tolerance of the hybrids were divided into three types, stronger, medium and softer. Five genotypes, Xianyu 335, Zhengdan958, Suyu2, Fuyou9, Liyu13 were with stronger drought tolerance. Twenty genotypes, Jidan50, Shandan 20, Jiyu 9, Xundan20 and Shenyu17, et al were with medium droughr tolerance. Twenty six genotypes, Shan902, Shandan308,Shendan16 and Yuyu22, et al showed the worst.2. Five indexes P_n,φPSⅡ, chlorophyll content, dry matter and leaf area were screened based on thirty three morphological and physiological traits with quantitative analyzing methods. The comprehensive drought resistance of maize varieties were calculated by the subordinate degree of drought index of screen, it was used to drought resistant sequencing in silking date, the two sequencing in accordance with the results of drought index.3. The response of plant growth, gas exchange and chlorophyll fluorescence parameters were studied in two different maize hybrids Zhengdan958 （drought resistance） and Shaandan 902 （drought-sensitive） under three different drought stresses （mild drought, moderate drought, severe drought ） and normal irrigation in pot experiment. The results showed that drought stress inhibited the growth of two maize plant growth and the relative growth rate, resulting in a significant decline in biomass. With the increasing degree of drought stress, the maximum leaf net photosynthetic rate （Pnmax）, apparent quantum efficiency （AQY）, light saturation point （LSP）, stomatal conductance （Gs）, stomatal limitation （Ls）, maximum electron transport rate （ETRm）, photosynthetic efficiency （α）, PSⅡactual quantum yield （φPSⅡ） and photochemical quenching （qP） were decreased, while the intercellular CO₂ concentration （Ci）, light compensation point （LCP） and qN were increased. But the change extents of all parameters were smaller in Zhengdan958 than in Shandan902. This finding indicated that drought stress could significantly decrease the biomass of two maize varieties, possibly caused by reduction in the photosynthetic efficiency of plants. The drought stress damaging effects of plant growth and photosynthesis was minimal on the Zhengdan958 compared to Shaandan902. Under drought stress the Zhengdan958 maintained higher photosynthetic efficiency, stronger light energy transfer capacity and greater relative growth rate may be the major physiological traits in the adapt ability to drought conditions.4. Compared with the control, the root shoot ratio was raised in drought-sensitive maize. The rate of root activity of drought-tolerant maize reduced more than drought-sensitive maize. Root SOD activity was higher than that in the control plant. In addition, root MDA content increased but root soluble protein content decreased in maize. To the increase rate of MDA content and reduced degree of root soluble protein content, drought-sensitive maize were higher than drought-tolerant maize. In a word, root activity, SOD activity and root soluble protein content of drought-resistant maize were high. So these can slower root aging process and extend the period of root function, which may be one of the important role in getting high yield of drought-tolerant maize under drought stress.5. Effects of anthesis water stress on photosynthesis, senescense of leaf and matter redistribution in maize. Water stress significantly reduced stomatal conductance, transpitation rates and photosynthetic rate, and Shaandan902 decreased much faster than Zhengdan958. Actual photosynthetic efficiency, and photochemical quenching were much lower, while non-photochemical quenching were much higher under water stress than in control, indicating damage to photosystemⅡ. Compared with CK, water stress reduced the activities of protective enzymes as superoxide dismutase （SOD）, catalase （CAT） and peroxidase （POD）, while increased the content of malondiadhyde （MDA）, thus enhanced membrane lipid peroxidation. In addition, significant relativities between root activity, root weight, catalase （CAT）, the content of malondiadhyde （MDA）. Those revealed that depressed anti-oxidative enzyme activities, and leaf photosynthetic characteristics were key physiological mechanisms in affecting the grain yield formation under anthesis water stress.6. Effects of anthesis water stress on grain yield in maize. Water stress significantly reduced grain 1000-kernel weight, grain yield. The obvious increase in the amount of pre-anthesis dry matter accumulated, translocation accumulated and contributed to the grain dry matter under water stress. While water stress reduced harvest index. In conclusion, decreased dry matter accumulation translocation were involved in the depressed grain yield in maize under water stress.7. Effects of nitrogen on the dry matter weight, nitrogen accumulation and distribution under soil water stress. Soil water stress significantly reduced dry matter weight, but increased the distributive indexes in root, and increased the Root/Shoot ratio. Nitrogen enhanced the compensative capacity.The dry matter weight and WUE were all highest at 225 kg N ha^-1 level under water stress. P_n, stomatal conductance（G_s） and T_r were declined when the soil relative water content went down under soil water stress. Nitrogen increased P_n, G_s and T_r, in maize leaf under soil water stress. These results suggest that 225 kg N ha^-1 is the optimal nitrogen application rate under soil water stress in our experiments. F_v/F_m, the quanttun yield of electron transport（φPSⅡ）, and the photochemical quenching co-efficient（qP） in maize leaf were all higher at 225 kg N ha^-1 than other two nitrogen level. 225 kg N ha^-1 was the best nitrogen application level for improving Pn among three nitrogen levels. The antioxidant enzymes activities were highest, MDA content were the lowest. The deficient(0 kg N ha^-1) and excessive nitrogen supply(450 kg N ha^-1) are of disadvantaged to the yield formation.