Photo-Physiological Response Of Maize To High Plant Density Under Integrated Water And Nitrogen Management

Increasing density is an effective measure to improve yield per unit area.How to optimize the adaptability of photo-physiological characteristics to dense planting for improving yield with limited supply of water and nitrogen is a scientific problem urgently needed to be solved at present.In Hexi oasis irrigated areas with high annual evaporation and the insufficient rainfall,agricultural production has a large input of water and nitrogen resources,and the potential of dense planting is not fully exploited.A field experiment was conducted in 2020-2021,with a split-split plot arrangement of treatments.Two irrigation levels on local traditional irrigation level（high,W2）and reduced by 20%（low,W1）formed the main plots;two levels of nitrogen fertilizer at a local traditional nitrogen level（high,N2）and reduced by 25%（low,N1）formed the split plots;three planting densities of local traditional density（D1）,medium（D2）,and high（D3）formed the split-split plots.In this study,we aimed to explore the compensation effect of increasing density on decreasing yield caused by water and nitrogen reduction,to clarify the feasibility of improving maize density tolerance by water and nitrogen management.The relationship between photosynthetic physiological characteristics and water and nitrogen management of maize at population,leaf and molecular levels were explored,and reveal the photo-physiological mechanism of water and nitrogen management improving maize density tolerance.The main results are as follows:（1）Compared to traditional irrigation and nitrogen levels,a reduction of 20%irrigation and 25%nitrogen application decreased grain yield,kernel number per spike,and 1000-kernel weight of maize.Compared to traditional density,high and medium density decreased kernel number per spike and 1000-kernel weight,but boosted grain yield via the increase of the spike number.When traditional irrigation and nitrogen fertilizer were reduced by 20%and 25%,increasing density could compensate for the yield reduction caused by simultaneous reduction of water and nitrogen.Water and nitrogen management alleviated the decrease of kernel number per spike and 1000-grain weight of maize with medium density,and improved the maize density tolerance,which resulted in compensation effect,and increasing density effectively played the potential of yield increase.The grain yield was increased by 19.1%to 41.6%,the spike number was increased by 28.0%～28.2%with a 20%reduction in irrigation and traditional nitrogen application and medium density（W1N2D2）,compared to a 20%reduction in irrigation and 25%nitrogen application and traditional density（W1N1D1）,which showed that the yield reduction caused by synchronous water and nitrogen reduction could be compensated by increasing density with the condition of the reduction of 20%irrigation and traditional nitrogen application.The spike number was increased by 23.9%to 25.7%,but the kernel number per spike was decreased by 9.1%to 11.7%,and 1000-kernel weight have not significant difference with W1N2D2,compared to W2N2D1,the negative effect of decreasing grain number per spike was compensated by increasing spike number,and grain yield was increased by 13.0%to 14.9%.（2）Compared to traditional irrigation,a 20%reduction of irrigation reduced the accumulation of photosynthetic sources and dry matter from early filling stage to maturing stage,and increasing density enlarged photosynthetic sources and promoted dry matter accumulation at late growth stage.A reduction of 20%irrigation and 25%nitrogen application with traditional density resulted in the negative effect of decreased photosynthetic physiology of maize population.Increasing density can maintain higher leaf greenness,leaf area index（LAI）and population growth rate（CGR）at late growth stage of maize,under a 20%reduction in irrigation and traditional nitrogen application,which provided the population photo-physiological basis to compensate for the negative effect of simultaneous reduction of water and nitrogen.Compared to W1N1D1,W1N2D2 increased leaf greenness,LAI,and CGR by 6.2%to 7.7%,64.4%to 83.1%,and 19.8%to 80.7%,respectively.It indicated that increasing density compensated for the negative effects of both water and nitrogen reduction on LAI and CGR,under the condition of 20%water reduction and traditional nitrogen application.W1N2D2treatment had greater leaf greenness,LAI,CGR,and NAR by 4.0%to 19.0%,20.3%to 23.2%,18.6%to 41.8%,and 4.6%to 21.1%of maize than W2N2D1,respectively.W1N2D2 treatment kept higher photosynthetic source and dry matter accumulation rate of maize at late growth stage,which was beneficial to yield increase.（3）A reduction of irrigation with traditional nitrogen increased thylakoid membrane area,enlarged photochemical reaction site,improved light energy storage,alleviated the decrease of chlorophyll content and net photosynthetic rate（Pn）.A reduction of 20%irrigation and 25%nitrogen,and increasing density resulted in the decrease of individual photosynthetic capacity of maize.Chlorophyll b（Chl b）and Chl a/b were decreased by a reduction of 20%irrigation compared to traditional irrigation,and the decrease of Chlorophyll a（Chl a）was less than that of Chl b with a reduction of 25%nitrogen compared to traditional nitrogen and medium over traditional density.Compared to W2N2D1,the content of Chl b in W1N2D2 was reduced by4.4%to 5.0%,but the stack of thylakoid（Chl a/b）was increased by 5.0%to 10.7%.It indicated that the site for photochemical reaction and light energy storage could be increased,and alleviated the decrease of photosynthetic physiological parameters of maize leaves by increasing planting density moderately under the condition of 20%reduction in irrigation and traditional nitrogen application.（4）Compared to traditional irrigation,a reduction of 20%irrigation increased heat dissipation（NPQ）,the actual photochemical efficiency（Y（II））,and the quantum yield of regulatory energy dissipation（Y（NPQ））of maize PSII.Y（II）and Y（NPQ）were increased with a reduction of nitrogen and medium density over traditional nitrogen and density.A reduction of 20%irrigation and 25%nitrogen,and increasing density resulted in the increase of ineffective loss of maize light energy as thermal energy,but improved the ability of light protection and weak light utilization.Under a 20%reduction in irrigation and traditional nitrogen application,increasing density can increase the Y（II）,NPQ,and Y（NPQ）of maize PSII,which formed the superposition effect of dense planting on low light utilization and its own photoprotection ability of maize.Reduced 20%irrigation had greater Y（II）,NPQ,and Y（NPQ）by 2.7%to 3.2%,12.9%to 14.8%,and 2.5%to17.7%than traditional irrigation.The Y（II）and Y（NPO）values at a 25%reduction of nitrogen were 3.1%to 3.2%and 9.6%to 16.7%lower than those at conventional nitrogen application rate,respectively.The NPQ and Y（NPQ）value of medium density were increased by 5.8%to 8.2%and 3.4%to 9.7%,respectively,compared to traditional density.The results showed that the 20%reduction of irrigation combined with medium density enhanced the utilization of low light and photoprotection ability.Compared to W2N2D1 treatment,the maximum photochemical efficiency（Fv/Fm）and Y（II）of W21N2D2treatment were increased by 1.3%to 1.4%and 5.3%to 5.9%,respectively,but the quantum yield of non-regulatory energy(Y（NO）was decreased by 7.5%to 12.5%,it suggested that W1N2D2 treatment can reduce photodamage and promote electron transport,and such improving photochemical ability.（5）A reduction of 20%irrigation decreased PEPC enzyme activity and relative gene expression,but increased the enzyme activities of NADP-MDH,NADP-Me,PPDK and Rubisco in comparison to traditional irrigation.The enzyme activities of PEPC,NADP-MDH and Rubisco were decreased by a reduction of 25%nitrogen compared to traditional nitrogen application.High and medium densities decreased ATP content,NADP-Me,PPDK and Rubisco enzyme activities compared to traditional density.The key enzymes involved in carbon assimilation were decreased by a reduction of 20%irrigation and 25%nitrogen and increasing density.These formed the photo-physiological mechanism that water and nitrogen management can improve the carbon assimilation level of maize density tolerance.Compared to W2N2D1,W1N1D1 decreased PEPC,NADP-MDH,and Rubisco enzyme activities,and decreased pepc and rcb gene expression,it indicated that the simultaneous reduction of water and nitrogen leaded to the decrease of carbon assimilation capacity.W1N2D2 treatment increased ATP content,increased PEPC and Rubisco enzyme activities,and increased the expression levels of pepc,nadp-mdh,and rcb,it indicated that the 20%reduction in irrigation and traditional nitrogen application with medium density can alleviate the negative effects of simultaneous reduction of water and nitrogen on the activities of key enzymes and gene expression levels of carbon assimilation.W1N2D2 treatment increased ATP content,decreased the PPDK and Rubisco enzyme activities in comparison to W2N2D1.The high ATP content provided sufficient energy for carbon assimilation,and the improvement of the activities of other enzymes related to carbon assimilation compensated for the decreased activities of PPDK and Rubisco enzymes from the early filling to maturing stage,and alleviated the decline of CO₂assimilation ability.