Effects Of Increasing Planting Density And Decreasing Nitrogen Rate On Yield Formation Of Ummer Maize And It Phusiological Mechnisms

In corn production, how to set reasonable planting density and N application level to increase crop yield is not only the need of theoretical research, but also has important guiding significance in the production practice. Field experiments were conducted during the growing season of 2013 and 2014 at Experimental Farm of Shandong Agricultural University and State Key Laboratory of Crop Biology. Two high-yield summer maize（Zea mays L.） cultivars, Denghai618 and Denghai605 were selected for testing. Treatment levels consisted of（i） two different plant densities: 67,500 pl·ha-1（LD） and 97,500（HD） pl·ha-1, and（ii） four different rates of N application: 0（N0）, 180（N1）, 360（N2）, and 540 kg N ha-1（N3）. Our research objectives were to test the high yield potential of summer maize. Meanwhile, a combination of physiology and proteomics research approach was organized for further investigation of both the physiological mechanism and the molecular mechanism of high-yield summer maize under high density, which will provide theoretical basis and technical supports for high-yield and high-efficiency cultivation. The main results were as follows: 1 Increased density and reduced N rates could improve dry matter accumulation and grain yield of summer maizeProductivity per plant, kernels per spike and thousand kernel weight were induced by increasing density, however, grain yield and total dry matter accumulation were improved significantly due to the larger population under HD. Grain yield, total dry matter accumulation, kernels per spike and thousand kernel weight all showed a trend of first increasing then decreasing as the N application rate decreased. Excessive N has counter-productive effects to obtain high yield.Crop growth rates were improved under the condition of increased density and reduced N rates at each growth stage, which were beneficial for the accumulation of dry matter and hence provided sufficient material basis for yield formation.The effects of decreasing nitrogen application on yield were less under low density than high density. A high yield could be obtained by either increasing planting density to 97500 pl hm^-2 or decreasing N application to a level of 180 kg hm^-2（LD） or 360 kg hm^-2（HD）. The highest yield under high density and 360 kg hm^-2 N level showed that population yield potential was given full play and a balance between crop demand and N supply could be achieved by increasing density and decreasing the application of N fertilizer. 2. Increased density and reduced N rates could coordinate C and N translocation of summer maizeIncreasing planting density and reducing nitrogen supply appropriately were conducive to the accumulation of dry matter and distribution to grain. Density and N application could lead to different patterns of allocation of ¹³C-photosynthates. Reducing nitrogen application promoted the distribution of ¹³C-photosynthates to grain, increased the translocation efficiency of dry matter in vegetation to grain.Increasing planting density and reducing nitrogen supply improved aboveground total N accumulation at tasseling stage and mature stage and post-tasseling N accumulation, which were convenient to the satisfied different organs of nutrition requirement and laid the foundation of increase production further.Appropriate nitrogen supply under high density could reduce N translocation in stem and leaf organ to maintain a higher nitrogen accumulation in vegetative organs and to prevent leaf senescence with the purpose of increasing the formation of photosynthate.Increased density and reduced N rates also improved recovery efficiency（NUE）, agronomic efficiency（AEN） and partial factor productivity（PFPN） of applied N significantly. 3. Increased density and reduced N rates could enhance photosynthetic performance of summer maizeThe effective area of photosynthesis, leaf area index and canopy apparent photosynthesis were improved significantly owing to the increased density and net photosynthetic rate, canopy apparent photosynthesis, gs and chlorophyll content were increased under the condition of reducing nitrogen supply appropriately, which made contributions to optimize the canopy structure. Besides, the activities of PEPC and Rubisco were improved by N application, and changes of proteomics were induced at the same time. There were 37 photosynthesis related proteins identified in our current study which almost involved every process of photosynthetic.Leaf area index, net photosynthetic rate, canopy apparent photosynthesis, chlorophyll content, activities of PEPC and Rubisco were improved under the density of 97500pl·hm^-2 and N application of 360 kg·hm^-2. In addition, processes of leaf light capture, ATP synthesis, CO2 fixation and the Calvin cycle were affected by N supply, and the whole photosynthetic process were affected subsequently. Moreover, higher photosynthetic performances were maintained to improved radiation use efficiency and leaf senescence were delayed at the later growing stage due to the N application. It gives full play to the population photosynthesis production potential, and provides guarantee for the formation of grain yield. 4. Nitrogen regulated aging process of summer maize under high densityThe activities of SOD and POD in maize leaves were significantly improved and a lower membrane lipid peroxide level occurred with the N-fertilizer application which delayed leaf senescence.Leaf senescence is closely related to the function of the protein level change. In this study, we identified 32 ―senescence-associated proteins‖, which were down-regulation during leaf senescence. Aging process directly affects the function of proteins, and thus affect the physiological processes involved.There were 71 ―nitrogen-regulated proteins‖ identified in our study, and most of them were up-regulation in N2 treatment compared to N0 treatment. Nitrogen affected proteins involved in energy, disease/defence, protein synthesis, metabolism, transcription, protein destination and storage, secondary metabolism, signal transduction, cell growth/division and other proteins and affected the physiological changes ultimately.Moreover, we discovered, for the first time, 29 ―nitrogen-regulated senescence proteins‖ had significant interaction term for nitrogen × stage and proved that nitrogen could control the corn leaf senescence at the protein level. The degradation of senescence-associated proteins involved in many biological processes, especially energy, metabolism and cell rescue, defense and virulence pathways, which processes were regulated by nitrogen during leaf senescence...