Effects Of Density And Nitrogen Fertilizer On Leaf Senescence And Carbon And Nitrogen Metabolism Of Summer Maize At Later Growth Stages In Hilly Region Of Sichuan

It is an important period for maize grain yield formation after silking,and the physiological basis for improving maize yield is to delay leaf senescence,increase photosynthetic product accumulation and enhance carbon and nitrogen metabolism.In this study,Zheng Hong No.6,a dense-resistant maize hybrid suitable for machine harvesting,was used as the material,and the two-factor splitting area test design was adopted.Three density levels:constant density（A1:52500 plants/hm²）,densification1（A2:67500 plants/hm²）and densification 2（A3:82500 plants/hm²）;Four nitrogen fertilizer levels:total nitrogen treatment（B3:240kg/hm²）,nitrogen reduction by 25%（B2:180kg/hm²）,nitrogen reduction by 50%（B1:120kg/hm²）,and no-nitrogen treatment（B0:0kg/hm²）,the effects of densification and denitrification on material production and accumulation,senescence of post-flowering ear leaf and carbon and nitrogen metabolism were studied.The main results obtained are as follows:1.Moderate densification can increase the effective panicle number and leaf area index to a certain extent,thus increasing the dry matter mass accumulation of the population and finally achieving the goal of increasing yield;However,over-dense planting will affect ontogeny,the accumulation of dry matter per plant will decrease,the traits of panicle will become worse,and the stem is fragile and easy to fall,so it is not conducive to high yield.Increased application of nitrogen fertilizer can promote the growth and development of corn,resist overturning of strong stems,increase the area of green leaves,delay the senescence of late leaves,and increase the accumulation of dry matter,so it is beneficial to increase the yield of corn.However,under the circumstance that the total amount of nitrogen fertilizer is too much,an appropriate amount of nitrogen reduction will not significantly affect the growth and development of plants and yield reduction.Under the conditions of this experiment,higher yield and economic benefits can be obtained by increasing density by 1（A2:67500 plants/hm²）and reducing nitrogen by 25%（B2:180kg/hm²）,which is conducive to the green,efficient and sustainable development of maize production in this area.2.With the increase of density,the activities of CLH and pheophyllase（PPH）in panicle leaves increased,the peak was earlier,and the chlorophyll content decreased earlier and faster.Under appropriate nitrogen fertilizer conditions,CLH and PPH activities of panicle leaf were relatively low,and chlorophyll content decreased slowly,which was beneficial to prevent premature senescence of panicle leaf.The CLH activity was 8.36 times higher than that of PPH in each period,indicating that CLH is a key enzyme for chlorophyll degradation in maize leaves.The dynamic attenuation process of the relative green ear-leaf area after silking conforms to the Logistic equation y=a·e^b-cx/(1+e^b-cx),and the senescence starts on the day 25.07-32.91d after flowering,which is basically consistent with the change of chlorophyll content.Both planting density and nitrogen fertilizer level significantly affected the onset and time of maximum aging rate,maximum aging rate（Vmax）and average aging rate of leaves.Systematic clustering analysis shows that A1B3 is the slowest aging type,A2B2 is the slower aging type,A3B1 is the faster aging type,and A3B0 is the fastest aging type.3.With the increase of density,the activity of Rubisco and PEPC enzymes in panicle leaf decreased,photosynthetic capacity decreased,carbohydrate synthesis was affected,and the coordination degree of carbon and nitrogen metabolism decreased.Adequate supply of nitrogen fertilizer guarantees the supply of nitrogen in plants,prolongs the leaf function period,improves the activity of photozyme,and promotes the production of photosynthetic materials.It is also beneficial to promote the expression of Zm SPS1 and Gln1-3 genes,improve the activity of carbon and nitrogen metabolic enzymes SPS,GS and GOGAT,and better coordinate the relationship between carbon and nitrogen metabolism.In this study,the difference between nitrogen reduction of 25%under normal density（52500 plants/hm²）and densification 1（67500 plants/hm²）and the total nitrogen treatment was small,the carbon and nitrogen metabolic enzyme activity was higher,and the carbon and nitrogen coordination was better.Therefore,the proper densification and densification were beneficial to the stable growth,development and quality of plants on the basis of stable yield and reduced consumption.To sum up,summer maize production in the hilly areas of central Sichuan should be properly densification and nitrogen reduction under the original production conditions（density 52500 plants/hm²,nitrogen 240kg/hm²）,that is,density increase of 15000 plants/hm² to 67500 plants/hm²,nitrogen reduction of 25%is nitrogen180kg/hm²,not only conducive to coordinate the balance of source and storage;It can also delay the senescence of the leaves at the spike position after summer maize,coordinate the balance of carbon and nitrogen metabolism,and achieve the purpose of reducing consumption,maintaining quality and stabilizing yield.