中文摘要：

为探讨乙矮合剂调控夏玉米氮素同化和防止后期早衰的生理机制,为建立华北夏玉米区密植高产稳产化学调控技术提供理论依据,2013—2014年在中国农业科学院新乡试验站,以中单909和浚单20为材料,设置乙矮合剂（ECK）和密度梯度处理,研究密度梯度对花后玉米穗位叶氮同化特征和早衰的影响,以及ECK的化学调控效应。结果表明,灌浆期（花后0-40 d）,穗位叶硝酸还原酶（NR）活性和谷氨酰胺合成酶（GS）活性随密度增加而显著下降;灌浆后期（花后30-40 d）,谷草转氨酶（GOT）和谷丙转氨酶（GPT）活性随种植密度增加而显著降低。穗位叶叶绿素相对含量、可溶性蛋白含量和游离氨基酸含量在灌浆中后期（花后20-40 d）随种植密度增加而显著降低;两品种产量在7.5万株hm–2密度达最大值,7.5-10.5万株hm–2密度群体产量下降,高密群体易发生早衰。ECK处理显著提高了各密度群体灌浆中后期（花后20-40 d）穗位叶NR活性、GS活性、游离氨基酸含量、可溶性蛋白含量和叶绿素含量;显著提高了高密群体（7.5-10.5万株hm–2）GOT活性和GPT活性;较高密群体下（7.5-10.5万株hm–2）,中单909和浚单20较各自对照的增产幅度分别为5.59%-6.63%和6.73%-8.10%。ECK处理提高了高密群体夏玉米穗位叶片氮代谢关键酶活性及其产物含量,保证密植群体氮代谢正常进行,有效防止早衰及提高产量。综上所述,采用合理的种植密度并结合喷施乙矮合剂可作为华北夏玉米区高产栽培的重要技术措施。

英文摘要：

To explore the effects of ethylene-chlormequat-potassium （ECK） on the physiological mechanisms of nitrogen assimilation and early senescence in summer maize, and provide a theoretical basis for high and stable yields of summer maize under high plant density by chemical regulation technology in North China Plain, we conducted a field experiment using two varieties of Zhongdan 909 and Xundan 20 with different planting densities in 2013 and 2014. The results showed that increasing plant density significantly decreased the activities of nitrate reductase （NR） and glutamine synthetase （GS） after anthesis, glutamate oxaloacetic transaminase （GOT） activity at anthesis, and glutamate pyruvate transaminase （GPT） activity at 30 days after anthesis. The soluble protein and free amino acid contents at 20 days after anthesis, and the SPAD value at 10 days after anthesis were significantly decreased with increasing plant density. For both varieties, grain yield reached the highest under 7.5× 104 plants ha-1, while decreased under the density of 〉 7.5 x 104 plants ha-1. The higher density resulted in early senescence. ECK treatment significantly increased the activities ofNR, GS, and contents of free amino acid, soluble protein and SPAD value during middle and late grain filling stages. ECK treatment also significantly improved the GOT and GPT activities under 7.5× 104-10.5× 104 plants ha-1. Under the density 〉 7.5×104 plant ha-1, the yields of Zhongdan 909 and Xundan 20 compared with their own control were respectively increased by 5.59%-6.63% and 6.73%-8.10%. ECK treatment significantly increased the nitrogen assimilation key enzyme activity of ear leaf and its product content under different planting densities, promoted normal nitrogen assimilation and prevented early senescence under higher planting densities, therefore increased yield. Thus, an appropriate planting density combined with ECK application could be an important technique for achieving high grain yield for summer maize production in