Research On Dense Planting Effects And Density Tolerance Mechanisms In High Yield Summer Maize And N-fertilizer Regulation

Increasing plant density is one of the primary measures to achieve high yields in modern maize production. However, it seems very likely that grain yield decrease per plant and propensity to stalk-lodging with the planting density increase, which plays a part in reducing realizable yield potential in this crop. Using density-tolerance maize hybrid can effectively reduce the negative effects brought by high density cultivation. Field experiments were conducted during the growing season of 2014 and 2015 at Huang-huai-hai Corn Science and Technology Innovation Center of Shandong Agricultural University and State Key Laboratory of Crop Biology. Two widely planted cultivars, Zhengdan958（a compact and density-tolerance cultivar） and Ludan981（a sparse planting and big spike type cultivar）, were selected as the experimental plants（henceforth referred to as “ZD958” and “LD981” respectively）. Two planting densities（52 500 plants·ha-1, D1 and 82 500 plants·ha-1, D2） were set to study the response differences of different density-tolerant maize hybrids to plant density increase in morphology, physiology and proteomics characteristics. In addition, three nitrogen rates（0, 180, 360 kg·ha-1, designated N0, N180 and N360, respectively） were set up in this experiment to study the effect of nitrogen rate on these indexes under high plant density. The main results were as follows:1. Effects of plant density and N rate on the graon Yield and its composition of Summer MaizeIncreasing plant density can improve the grain yield of two hybrids, but the increament of the two hybrids was different. The grain yield of ZD958 were increased by 33.7% and 33.1 in 2014 and 2015 growing seasons, respectively, while that of LD981 were increased only by 3.2% and 4.9%. The kernel number per ear and 1000-kernel weight of both bybrids reduced with the plant density increased, but the decrease amplitude of 1000-kernel weight was much lower, indicating that plant density affected grain set mainly by affecting the kernel number per ear. No significant differences were observed between the two hybrids in grain set under the high plant density. Thus, the higher lodging rate and less harvested ears were the main reason for the less increase in grain yield of LD981 under high plant density. Under the condition of high density, N supply can improve the ear numbers, kernel number per ear and 1000-kernel weight of both bybrids, and then increased the grain yield of both hybrids. Plant density increase and N supply significantly improve the total dry matter accumulation and dry matter accumulation after silking, and no significant differences. The lower harvest index was one of the important reasons for the lower grain yield in LD981 under high plant density.2. Effects of plant density and N rate on the lodging resistance of summer maizeIncreasing plant density can improve the plant height, ear height, the length of the third basal internode in summer maize, but significantly reduce the stem diameter, RPS, BS, rind thickness, vascular bundle sheath′s thickness and number of vascular bundle of both hybrids. The changes of these stem traits lead to a high lodging rate. Take the 2014 growing season as an example, the lodging rates of LD981 and ZD958 uner high plant density were 42.8% and 14.6, respectively. Compared with LD981, the lower plant height, ear height and ear ratio and higher RPS, BS and number of Vb of ZD958 resulted in less stalk lodging at the high density. Meanwhile, N supply might improve the stalk quality and decrease the risk of stalk lodging, however, this cannot compensate too high increase of lodging rate under the high plant density for the lodging susceptible hybrids. In addition, in stepwise regressin analysis, the variability of lodging resistance in maize was demonstrated to be due in part to variability in ear height and rind thickness.3. Effects of plant density and N rate on the photosynthetic capacity of summer maizeThe increased leaf area index and reduced canopy light transmittance can significantly improve the light use efficiency under high plant density. However, the chlorophyll content, net photosynthetic rate and photosynthetic enzymatic activities significantly decreased with plant density increased indicateing that the light distribution was not reasonable under the high density, which resulted in reduction in photosynthetic performance. N supply can improve the chlorophyll content, net photosynthetic rate, photosynthetic enzymatic activity of summer maize at high plant density, improving the plant photosynthetic performance. The lower bottom light transmittance, canopy closure, faster senescence speed and lower leaf chlorophyll content, net photosynthetic rate and photosynthetic enzymatic activities of LD981 were important reasons for the lower photosynthetic performance and no significant increase in grain yield under high plant density.4. Effects of plant density and N rate on ¹³C-photosynthates distribution in different organsIncreasing plant density led to different patterns of allocation of ¹³C-photosynthates. At R1, the maximum ratio of distribution of labeled photosynthates was recorded in the stem, however, the ratio of ¹³C-photosynthates in stem decreased with plant density increased, and the reduction of LD981 were larger. Moreover, N supply can significantly improve the transport of ¹³C-photosynthates from ear leaf to other organs. After that, they were retranslocated to the grains during the latter part of the grain filling period. Increasing plant density can reduce the allocation of ¹³C-photosynthates in grains at R6.5. Effects of plant density and N rate on the ratio of carbon to nitrogen and N absorption, transmission and utilization efficiencyThe activeness of enzymes in carbon and nitrogen metabolism were significantly reduced with plant density increased In addition, N supply significantly reduced the ratio of carbon to nitrogen of leaf and stem sheath at high density of both hybrids. The N translocation amount and translocation efficiency were significantly increased at high plant density, and the higher translocation efficiency was an important reason for the rapid senescence of LD981 under high density. Although, N translocation amount increased with N rate increased, the N translocation efficiency decreased. The Recovery of N fertilizer, N agronomic efficiency and N partial factor productivity were increased with plant density increased, but decreased with N rate increased.6. Effects of plant density on root growth of summer maizeThe roots of ZD958 prove to be less affected by plant density. At high density, it can maintain relatively high root weight, length, absorbing area and activity, and accompanied by a long high value duration and a slow senescence speed in the late growth stage, which was beneficial to N uptake, the photosynthetic production and obtaining higher grain yield. This suggests that well-developed roots can guarantee the plant nitrogen uptake at high density, contributing to the photosynthetic production of the aboveground part and thus achieving higher grain yield. However, the size of the roots is not a limitation affecting the plant nitrogen absorption, which is simultaneously regulated by the growing demands of the aboveground part.7. Effects of plant density on the differential proteomics of ear leaf of summer maizeThe differential proteomics of leaf in two hybrids affected by plant density increase, according to the main function, can be distributed in 4 areas: photosynthesis, metabolism, stress and defense and protein synthesis/assembly/degrade. The number of protein spots of LD981 affected by plant density was much more than ZD958. There were 12 protein spots that were significantly affected by plant density increase in both hynrids, and their functions involved in photophosphorylation, Calvin cycle, C4 pathway, glycolysis, N assimilation and adversity stress response and other physiological processes. The expression abundance of photosynthetic phosphorylation, Calvin cycle, C4 pathway, glycolysis, nitrogen assimilation related protein of both hybrids were significantly reduced with plant density increased which were similiar to the reduction of photosynthetic performance and C/N metabolism under high plant density. The expression abundance of adversity stress related proteins were significantly increased under high density, which can reduce the impacts of plant density increase on maize plant. At the same time, all kinds of protein expression abundance of ZD958 were significantly higher than that of LD981, which might be the molecular basis of density resistance under high plant density...