Effects Of Difference In Ear Leaf Light Environment Of Densely Planted Summer Maize On Its Photosynthetic Performance And Proteomics Analysis

Increasing planting density is one important method to further improve the grain yield of summer maize.As planting density increases,shade generated by the plant canopy limits the extent of available light for within-canopy photosynthesis.In addition,in the field,plants are exposed to a dynamic light environment characterized by frequent alternations between high and low light intensities,which can be caused by cloud cover,wind,and shading from the upper leaves（self-shading）and those of neighboring plants.Due to a delay in photosynthetic induction after the transition from low light to high light,the loss of photosynthetic carbon fixation should be lower under fluctuating light than under steady-state light environments.As planting density increases,the fluctuations in light intensity perceived by maize ear leaves are more pronounced and the loss of photosynthetic carbon fixation is thus more serious.A large number of studies on fluctuating light have been carried out on understory plants or C3 crops,but rarely on densely planted C4 crops.This study was conducted at the Shandong Agricultural University Experimental Farm and the State Key Laboratory of Crop Physiology of Shandong Agricultural University,during the 2017 and 2018 growing season.ZD958,a high-yielding and density-tolerant variety grown extensively in the North China Plain,was used in this work.At the same Nitrogen application,maize plants were cultivated at four densities:low density（LD,22,500 plants ha-1）,medium density（MD,45,000 plants ha-1）,normal density（ND,67,500 plants ha-1）and high density（HD,90,000 plants ha-1）.Nitrogen application levels were all 225 kg N ha-1.We measured the photosynthetic potential,photosynthetic performance and yield potential of ear leaves of summer maize,combined the physiological,biochemical and proteomic to deeply explore the physiological,biochemical and molecular mechanisms of the response of densely planted summer maize to low light and fluctuating light,with a view to providing theoretical basis and technical support for high-yield and high-efficiency cultivation and resource-efficient utilization of summer maize in the Huanghuaihai region.The main results were as follows:1 Planting density significantly affected light environment of ear leaves in summer maizeAs planting density increases,the photosynthetically active radiation（PAR）received by ear leaves was significantly reduced.Under high density,the relative difference of PAR of ear leaves were higher than other treatments,and the relative PAR（%）more irregular.Therefore,the leaves of high density with lower light intensity and more pronounced fluctuations.2 Rational dense planting improves dry matter accumulation,grain yield,and nitrogen utilization efficiency of summer maizeIncreasing planting density significantly increased the dry matter accumulation of population and thus grain yield.Increasing planting density significantly increased the nitrogen accumulation aboveground,promoted nitrogen transport to grain and increased the contribution proportion of translocation nitrogen to grain（NTCP）.Increasing planting density significantly increased the assimilation amount of nitrogen after anthesis（ANA）and total nitrogen accumulation（TNAA）,which were convenient to the satisfied different organs of nutrient requirement and laid the foundation of increase production further.As planting density increases,nitrogen translocation amount（NTA）and partial factor productivity of nitrogen（PFPN）significantly increased.The nitrogen utilization efficiency（NUcE）of high density was significantly higher than that of low density.3 Rational dense planting improved low light utilization of ear leaves in summer maizeUnder high density,leaf mass per area（LMA）,leaf nitrogen content and chlorophyll content decreased significantly in each growth stage,but the apparent quantum efficiency（AQE）and the unit light use efficiency in the field were significantly higher than those in low density and normal density.Increasing planting density significantly improved the donor-and acceptor-side performance of photosystem Ⅱ（PSⅡ）reaction center,which enhanced the transfer of photosynthetic electrons from PSⅡ to photosystem Ⅰ（PSI）,and promoted the coordination between photosystems（Φ（PSⅠ/PSⅡ））of leaves after anthesis.The higher Φ（PSⅠ/PSⅡ）)improved the stability of photosynthetic performance during yield formation,maintaining a higher net photosynthetic rate（Pn）of leaves in the middle and late flowering period,and thus improving the light use efficiency.4 Rational dense planting improves the fluctuating light utilization of ear leaves in summer maizeUnder fluctuating light,increasing planting density significantly increased the photosynthetic induction rate（IS%）,which significantly improved photosynthetic recovery and significantly decreased the loss of photosynthetic carbon fixation.At 0 day after anthesis,with the increase of the fluctuation duration,there was no significant difference in the IS%of leaves under normal and high density.With the growth process,the IS%in high density was significantly higher than that in normal and low density.Therefore,the plants grown in high density had strong resistance to fluctuation,which significantly increased the photosynthetic accumulation after anthesis and improved the fluctuating light utilization efficiency.5 Proteomics mechanism of rational dense planting improving light energy utilization in summer maize leavesAt 0,20 and 40 days after anthesis,a total of 296 photosynthesis-related differentially expressed proteins were found in low,normal and high density ear leaves by using tandem mass tag（TMT）quantitative proteomics and these proteins are involved in multiple pathways during photosynthesis.Under high density,the synergistic improvement of light-harvesting pigment protein,photosystem Ⅱ and photosystem Ⅰ protein,and electron transport chain protein was beneficial to promote the light absorption,capture and transfer of summer maize leaves.With leaf senescence,the effect of planting density on photosynthesis gradually shifted from the light reaction stage to dark reaction stage,especially the Calvin-Benson cycle（C3 pathway）.Under high density,the expression levels of Rubisco significantly decreased;however,the relative expression levels（relative to Rubisco）of most photosynthesis-related proteins in C4 and C3 pathway were significantly higher than those in low density and normal density,which synergistically increased CO2 and ribulose diphosphate（RuBP）substrate levels of dark reaction,with conducive to maintaining a high photosynthetic initiation and recovery speed and thereby improving light utilization efficiency under fluctuating light conditions.Therefore,other proteins in carbon fixation,rather Rubisco,are the limiting factors for fluctuating light utilization in high density.The synergistic improvement of photosynthesis-related proteins in light reaction and dark reaction stage jointly promoted the utilization of low light and fluctuating light in high density leaves,and finally achieved the grain yield increase.