Effects Of Nitrogen Application On Ear Leaf Physiology And Yield Of Maize Under High Temperature Stress At Ear Stage

The aim of this study was to investigate the effects of different nitrogen application rates on photosynthetic physiology and yield of maize under high temperature stress at ear stage,and to guide maize to achieve stable yield under stress.We conducted field experiments at the Zhengzhou Agricultural Meteorological Observatory Comprehensive Test Station(34°43’N,113°39’E)of Henan Institute of Meteorological Sciences for two consecutive years in 2020 and 2021.Three factors split plot design was adopted,with temperature as the main zone,variety as the secondary zone and nitrogen level as the secondary zone.The field room temperature was set as the control,and the film covered scaffold was used for high temperature treatment.The widely cultivated heat resistant variety Zhengdan 958(ZD958)and heat sensitive variety Xianyu 335(XY335)were used as test materials.There were three nitrogen levels,which were low nitrogen N90(90 kg/hm2),medium nitrogen N180(180 kg/hm2)and high nitrogen N270(270 kg/hm2).We studied the effects of different nitrogen application rates on photosynthetic physiology,carbon and nitrogen metabolism,antioxidant physiology,dry matter accumulation of the panicle leaves and yield of maize and genotypes under high temperature stress.The results are as follows:(1)At the beginning of high temperature stress,the photosynthetic pigment content of the two cultivars did not change significantly,but the maximum photochemical efficiency(Fv/Fm),actual photochemical efficiency(YII),apparent electron transfer efficiency(ETR)and photochemical quenching coefficient(q P)increased.However,with the extension of time of high temperature stress,the decrease of heat sensitive variety XY335 was higher than that of heat resistant variety ZD958.In addition,reasonable nitrogen application could alleviate the decrease of photosynthetic pigment,net photosynthetic rate(Pn)and chlorophyll fluorescence parameters of maize leaves under high temperature stress,and N180 treatment showed the best performance,N270 treatment significantly inhibited photosynthetic performance.(2)With the prolongation of high temperature stress time,the activities of SOD and CAT increased first and then decreased,POD activity and MDA content were always higher than those of the control,and the degree of membrane lipid peroxidation was intensified.Rational nitrogen application increased the activities of SOD,CAT and POD,reduced the accumulation of MDA content,slowed down the lipid peroxidation of cell membrane,delayed the senescence of leaves,and enhanced the resistance to stress.In addition,compared with heat sensitive variety XY335,N270 treatment had a stronger inhibitory effect on POD activity of heat resistant variety ZD958 under high temperature stress.(3)Under high temperature stress,scientific nitrogen application could alleviate the decrease of the activities of nitrogen metabolism enzymes NR,GS and GOGAT and the decrease of soluble protein content in the leaves of the two cultivars.NR enzyme activity and soluble protein content increased gradually with the increase of nitrogen level,while GS and GOGAT activities decreased significantly under high nitrogen treatment.Compared with the control,GS and GOGAT of ZD958 leaves were significantly decreased by 43.2% and 57.8% under N270 treatment for 9 days,and XY335 was significantly decreased by 23.6% and 36.7%,respectively.Nitrogen had inhibitory effect on GS and GOGAT activities under high temperature stress.(4)High temperature stress increased the activities of sucrose metabolism enzymes SS and SPS,and the contents of soluble sugar,starch and sucrose in stem and leaf were also significantly increased to maintain osmotic balance in plants.Under high temperature stress,reasonable increase of fertilizer application could improve the activity of sucrose metabolism enzyme in maize leaves,and the content of NSC in stem and leaf,and enhance the stress resistance of maize plants.(5)Under high temperature stress,dry matter accumulation,total dry weight of aboveground and grain yield of two varieties of various organs decreased significantly.High temperature stress led to ear thinning and grain number reduction,which led to lower grain yield.Compared with the control,the grain yield of ZD958 and XY335 decreased the least under N180 treatment,significantly decreasing 46.1% and66.7%(2020),13.9% and 39.7%(2021),respectively,in which the decrease of heat sensitive XY335 was greater.Reasonable increase of nitrogen application rate could increase dry matter accumulation of maize organs,increase grain number per ear and increase grain yield.