Photo-ecophysiological Mechanism Of Density Tolerance For Intercropped Maize Response To Nitrogen Reduction Application

Intercropping has the advantages of high and stable yield,and resources efficiency.High density tolerance of intercropping is a key factor in its high yield,the proper photo-ecophysiological traits are the crucial foundations for improving this tolerance.With the increasing demand for fertilizer reduction,whether intercropping can maintain high density tolerance under nitrogen reduction condition and the photo-ecophysiological mechanism affecting intercropping density tolerance need to be further studied.From 2020 to 2021,a three-factor field experiment was conducted in Hexi irrigation area.The main plot is planting pattern（wheat and maize intercropping and maize solecropping）,the split plot is nitrogen application level（local conventional level N2,25%nitrogen reduction level N1）,the split-split plot is maize planting density（local conventional level M1;medium,33%densification,M2;high 66%densification M3）.The compensation effect of intercropping on nitrogen reduction through dense planting was studied to clarify the feasibility of intercropping to support maize density tolerance.The response characteristics of intercropping and solecropping to density increase were analyzed from population,individual and cell levels,and the photosynthetic physiological and ecological mechanism of maize density tolerance in intercropping under nitrogen reduction was clarified.The main research results are as follows:（1）Intercropping had the advantage of compensating the negative effect of nitrogen reduction on maize yield by dense planting,and higher biological yield,harvest index and ear number were the main reasons for the compensation effect.Compared with N2,N1reduced maize grain yield by 5.0%～8.4%;M2 and M3 increased maize grain yield by6.7%～13.4%and 2.4%～11.5%,compared to local conventional density（M1）,respectively;intercropping increased 34.1%～35.6%compared with solecropping.Under nitrogen reduction,IN1M2 had the highest grain yield among the treatments,16469～18812kg hm^-2,it improved grain yield of maize by 31.3%～45.9%compared to control（SN2M1）.IN1M2 increased biological yield by 37.1%～40.3%,harvest index by 5.6%～10.8%,and effective ear number by 57.0%～65.3%compared to the control,thus creating an intercropped maize density tolerance advantage.（2）In a certain range,increasing density could reduce the deterioration of photo-physiological characteristics of maize population caused by nitrogen reduction,and this effect of intercropping was higher than that of solecropping.Under reduced nitrogen conditions,the leaf area index（LAI）,leaf area duration（LAD）,crop growth rate（CGR）and net assimilation rate（NAR）of maize decreased by 3.3%～3.5%,3.3%～3.5%,5.1%～8.3%and 3.6%～5.5%.In late growth stage,M2 and M3 increased maize LAI,LAD by 10.0%～11.3%,21.3%～21.4%and 10.5%～13.2%,24.0%～30.3%,respectively,compared with M1;CGR and NAR of M3 was reduced by 9.7%～22.7%and 7.7%～23.1%compared with M1.intercropping increased maize LAI,LAD,CGR and NAR by 23.9%～26.1%,24.7%～25.1%,187.2%～205.4%and 76.6%～83.1%,compared to solecropping,respectively.IN1M2 improved LAI by 34.8%～37.3%,LAD by 31.3%～32.7%,CGR by 173.6%～185.5%,and NAR by 68.1%～126.1%compared with the control.It indicated that intercropping maize compensated for the negative effect caused by nitrogen reduction by dense planting and had a significant population photo-physiological advantage in the later growth period.（3）Intercropping weakened the decrease in photosynthetic performance of maize leaves caused by nitrogen reduction and dense planting compared with solecropping.N1decreased chlorophyll content（SPAD）of whole growth stage by 5.7%～9.3%,net photosynthetic rate（Pn）by 11.4%～15.9%,transpiration rate（Tr）by 9.5%～9.8%and actual photochemical efficiency Y（II）by 2.2%～5.8%.M2 and M3 reduced the SPAD of whole growth stage by 3.1%～3.4%and 5.0%～8.8%,Pn by 6.1%～7.6%and 9.0%～21.3%,Tr by 6.6%～7.3%and 12.9%～13.7%,and Y（II）5.8%～7.5%and 12.1%～12.2%respectively,compared with M1.In late growth stage,intercropping increased SPAD by5.7%～9.3%,Pn by 12.4%～20.8%and Tr by 9.8%～30.9%,respectively,compared with solecropping.IN1M2 maintained or weakened the decrease in SPAD,Pn,Tr and Y（II）of maize leaves in the late growth stages compared to the control.Thus,it created leaf photosynthetic advantage of dense tolerance in intercropped maize under nitrogen reduction conditions.（4）Intercropping alleviated the cellular damage and early senescence of maize leaves caused by nitrogen reduction and dense planting compared to solecropping.N1 reduced leaf soluble protein by 11.1%～11.4%,superoxide dismutase（SOD）activity by 9.0%～11.3%and catalase（CAT）activity by 12.3%～16.5%,compared with N2.M2 reduced leaf soluble protein at flowering by 9.4%～16.9%,CAT activity after flowering by 2.7%～5.3%,SOD was not significant compared with M1;M3 reduced averaged leaf soluble protein by9.4%～10.0%,SOD activity by 3.4%～7.4%CAT activity 8.0%～9.3%,and increased malonaldehyde（MDA）content 3.6%～7.6%.Intercropping increased the post-flowering leaf soluble protein by 13.3%～20.5%,SOD activity by 7.2%～11.5%,CAT activity by11.7%～12.2%,and reduced MDA content by 7.1%～8.0%during the reproductive period compared to solecropping.IN1M2 increased the soluble protein content,alleviated the degree of decline in SOD and CAT activities,and maintained MDA content compared with the control.Thus,it formed the advantage of cellular anti-senescence of intercropped maize under nitrogen reduction and dense planting.（5）Intercropping alleviated the deterioration of canopy ecological factors under dense planting compared to solecropping.N1 decreased the canopy air temperature difference by8.6%～25.1%,compared with N2.M2 and M3 reduced the averaged middle canopy PAR by 25.2%～26.6%and 36.7%～37.9%,and improved the canopy air temperature difference by 2.0%～3.5%and 5.9%～10.7%compared with M1,respectively.M3 reduced population CO₂ concentration by 0.7%～1.0%compared with M1.Intercropping significantly increased the middle canopy PAR of maize by 93.8%～185.2%,reduced the canopy air temperature difference by 45.6%～56.5%,and increased the population CO₂ concentration by 2.1%～2.2%compared with solecropping.IN1M2 effectively increased middle canopy PAR,reduced canopy air temperature difference,and increased population CO₂concentration compared with the control.Thus,it provided suitable canopy ecological condition for improving maize density tolerance.Therefore,intercropping can compensate for the negative effect of nitrogen reduction(nitrogen application of 270 kg hm^-2)on maize yield by increasing the maize planting density to 6.0×10⁴ plants hm^-2.The main reason is that under nitrogen reduction,intercropping can promote middle canopy PAR,canopy air temperature difference under dense planting,thus improving the cell membrane oxidation protective enzymes activities of maize leaves,avoiding the premature senescence of leaf cells after maize flowering,then improving the photosynthetic performance of leaves.Therefore,it can extend the functional period of leaves,promote the capacity of population photoassimilation,increase the biological yield and harvest index synergically,increase ear number to compensate for the decrease of kernel number per ear,and finally improve the density tolerance of intercropping maize under the condition of nitrogen reduction.This study provides a theoretical basis and technical support for the practice of high yield and efficiency of intercropping under resource-limited conditions.