Analysis Of Major Yield-limiting Factors Of Summer Maize And Regulatory Approaches In Heilonggang River Valley

Heilonggang River Valley (HRV) is a major grain producing region in China where the main cropping system here is the annual crop rotation of winter-wheat and summer maize. The summer maize is often subject to abiotic stresses such as drought during sowing time, high temperatures during corn silking, tasseling and pollination, lengthened raining days and lacks of sunshine, which are caused by seasonal meteorologic factors. Some other limiting factors, such as nitrogen application and plant density, will bring about problems like poor reproductivity of the high yields of some populations and difficulty in achieving full potentials. This study draws on ten year worth data of the field experiments in Wuqiao collected by a China Agricultural University research team, of which light, temperature and cultivation measures (planting density and nitrogen) are discussed when carrying out system analysis. This study quantifies summer maize’s full potential for yield and room to improve production, and analyses limiting factors for production and crucial cultivation measures impacting the real production. To address production limiting factors and issues of production, field control experiments have been carried out, which examine canopy structures, microclimate of canopy, photosynthetic characteristics, matter production, and yield formation in regard to two varieties of different ears, two planting densities and three row spaces. Supplementary measures such as supplementary pollination and delayed harvest have also been adopted for greater production. This study contributes its theoretical grounds and technical support to the production system of the region which aims to improve its yield and efficiency. The central findings were listed in the following:1. Affected by climatic changes, the yield potential of summer maize in this region on average decreases 55 kg·hm-2 annually, and the interannual yield potential fluctuates significantly (CV= 11.43%), due to the significant increase of annual temperature and the reduced days of sunshine in the growth period. Analysis of yield potential and yield from 2002 to 2013 demonstrates that yield has increased significantly with 63% yield potential achieved in experiment field and 44% achieved in farm field. The analysis of the correlation between sunlight and temperature and yield in different growth stages shows that sunshine and temperature have biggest impact on yield 15 days before or after silking, then the impact begins to decrease 15 days after silking until harvest and the impact is lowest between sowing and silking. Lack of accumulated valid photosynthetic radiation 15 days before silking to silking and high accumulated valid temperature between silking and 15 days after silking are major limiting factors in the formation of kernel number. When kernel number is below 4200 kernel m-2, insufficient accumulated solar radiation and temperature from silking to harvesting are the important factors in limiting the increase of 1000-kernel weight.2. The analysis of relationship between plant density, nitrogen application and grain yield and components of summer maize showed that with the increase of plant density, yield increased first (< 90 000 plants hm-2) and then declined. A strong linear (negative correlation) relationship was found between kernel number per ear,1000-kernel weight and plant density. The decline rates in 1000-kernel weight and kernel number per ear were 6.5 g and 16.51 kernels as the plant density was increase by 10 000 plants hm 2. There was a significantly quadratic relationship between grain yield,1000-kernel weight, kernel number per ear and nitrogen application amounts. The interacted effects of plant density and nitrogen on grain yield indicated that the optimum yield can be produced if plant density is 88800 plants hm-2 and nitrogen is 253.4 kg-hm2 in HRV.3. Optimized row spacing arrangement (60+45 cm) can construct a structure of a population with high yield and efficiency. The morphological structure of canopy will be improved:plants distribution across field was more even, plant height and ear height were lower, diameter of plant stem became larger, leaf area index (LAI) particularly at ear layer were increased, and high LAI can last for a longer time. Regarding microclimate within the canopy, light interception at ear leaf (17th leaf position from the bottom) decreased (<40%), and increased (35-37%) at leaf positions from 3rd leaf to ear leaf. Temperature at ear layer decreased, and meanwhile relative humidity increased. As far as photosynthetic performance was concerned, leaf net photosynthesis rate across whole plant, SPAD and Fv/Fm values increased, especially in leaves at the middle and bottom canopy layers. Accordingly, grain-filling rate increased, and the grain-filling period was extended, which results in an increased seed biomass.4. Nitrogen application regime can regulate canopy architecture and function of summer maize. Reduced nitrogen application at early growing stage (before 9-leaf stage) can reduce plant height and ear height, and therefore reduce the risk of lodging. Postponed nitrogen application (at silking stage) can increase photosynthesis rate and SPAD values during the grain filling period and delay leaf senesces, and therefore maintain leaf physiological activity at a higher level, and sustain LAI peak value for a lengthened time, which helps accumulate post anthesis matter, and contributes greatly to grain filling and yield.5. Regulation can significantly increase the yield of summer maize. The variety with a large ear size (DH605) can produce the higher gain yield at 75000 plants hm-2, while the variety with a medium ear size (ZD958) has a higher yield of 90000 plants hm-2. Compared with the equal row spacing (60+60 cm) at the optimal plant density, wide and narrow row spacing (60+45 cm) can increase grain yield of summer maize by 7.86%, to which increased kernel number contributes more than 1000-kernel weight. A suitable nitrogen regime, which applies nitrogen at the proportion of 2:5:3 during three stages i.e. sowing,9-leaf stage, and silking, can significantly increase grain yield of summer maize. Artificial pollination can increase the yield because the kernel number is lager, and it benefits the varieties with a large ear size more than the varieties with a smaller ear. A five days and a ten days delay in harvest will respectively increase yield 87 kg hm-2 d1 and 65.9 kg hm-2 d-1 over normal harvest. When stubbles were kept at the height of 35 cm, the average grain yields of three growing seasons were respectively 10.59% and 5.31%higher than when stubbles were kept at 15 cm and 25 cm.