| Maize（Zea mays L.）is the largest food crop and widely planted across China.It is of great importance to study and clarify the regional distribution characteristics,restricting factors of maize yield and the ways to improve the yield for ensuring food security in China and the world.In this study,a multi-year（2009-2016）and multi-site（40 sites）field experiment was conducted using common maize cultivars and densities in the four major maize growing regions across China,aiming at determining the spatial distribution characteristics and variations of maize yield in China,clarifing the contributions of maize dry matter production and distribution to grain yield,revealing the effects of changes in morphological characteristics on maize dry matter production and grain yield,exploring and evaluating technical approaches as well as its ecological effects to further increase maize yield in China.The main results were showed as follows:1.Interregional differences of maize dry matter production and distribution and its contributions to grain yieldOn average,the grain yield,dry matter production（DM）and harvest index（HI）were 11.09t ha-1,23.51 t ha-1,and 0.51 respectively,the contribution rate of DM was 73.71%,which was much higher than that of HI（26.28%）across China.Among the four maize growing regions,the contribution rates of DM in both NW（57.34%）and NM（64.46%）were higher than that of HI（42.66%and 35.54%,respectively）.In contrast,the contribution rates of HI in HM（52.67%）and SW（50.92%）were slightly higher than that of DM（47.33%and 49.08%,respectively）.There were significant differences in maize yield and dry matter production between different regions,which showed the same spatial distribution trend,i.e.,NW>NM>HM>SW.In comparison,the HI was in the order of NW>SW>NM>HM,which was different from the grain yield and DM.Therefore,the interregional differences of grain yield were mainly attributed to the differences of DM between regions.For the five different modern hybrids,the contribution rates of DM to grain yield were also significantly higher than that of HI at a large spatial scale.The significant differences in grain yield between cultivars were mainly due to the differences of HI.Additionally,combining the data of this study and data collected from previous studies,we found that the increase in maize grain yield under lower yield levels(<15t ha-1)was mainly attributed to the increase in both DM and HI,while under higher yield levels(>15 t ha-1)was mainly due to the increase in DM.2.Variations of maize main plant morphological characteristics and its effects on dry matter production and yieldThe mean final leaf number was 19.6 with the range of 16.7–23.3 across China.There was significant difference in the final leaf number between the different cultivars.The mean leaf number above the primary ear was 6.0 ranging from 4.7 to 7.7.The mean leaf number below the primary ear was 12.5 ranging from 10.0 to 15.3.The leaf numbers above and below the primary ear showed the same trends with the mean final leaf numbers for the same cultivars.The results of plant height traits analysis showed that the average plant heights of DH11,NH101,XY335,ZD958,and ZD909 were 305.3 cm,301.7 cm,301.5 cm,262.7 cm and 259.6 cm,respectively,the average ear heights were 128.7 cm,107.3 cm,111.4 cm,115.6 cm and 109.1cm,respectively,and the average ear ratios were 0.42,0.35,0.37,0.44 and 0.43,across all sites.According to the coefficients of variation,it was found that ear height was more sensitive to the environments than plant height and ear ratio.Furthermore,the mean internode length and mean internode length below the primary ear were found to be the main contributors to the differences in plant height and ear height,respectively.Overall,many climatic factors were found to significantly affect the final leaf numbers and plant height traits across four maize-growing regions in China.The results of stepwise regression indicated that the photoperiod was the key climatic factor that resulted in the variation of final leaf number,and the variations of both plant height and ear height were mainly affected by photoperiod and minimum temperature between regions,but the major climatic factors influencing ear ratio varied with cultivars.Finally,correlation analysis results showed that the variations of the final leaf number and plant height all had significant impacts on DM at the silking and physiological maturity grain yield at a large spatial scale.For every 10 cm increase in plant height,the DM at silking,DM at physiological maturity,and grain yield increased by approximately 0.41 t ha-1,0.87 t ha-1,and 0.38 t ha-1,respectively.3.Effect of increasing density on maize grain yield and environmental emission reductionThe comprehensive evaluation of regional close planting technology showed that maize grain yield could be increased by 5.59%across China and 10.5%,2.7%,5.2%and 10.3%respectively in SW,HM,NM and NW after the planting density increased by 15,000 plants per hectare without extra inputs of nitrogen（N）,which was mainly due to the increase in dry matter production.The increase in grain yield of different cultivars as the increase of density varied with the regions.The life-cycle assessment（LCA）showed that N surplus was significantly decreased by 11 kg N per ha,the reactive N loss intensity and GHG intensity decreased by 5.92%and 6.29%after the planting density increased by 15,000 plants per hectare,which ranged from2.6%to 10.1%and 2.2%to 10.2%,respectively between regions. |
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