| Breeding high-yielding and nitrogen (N) use efficient maize cultivars is one of key strategies to simultaneously ensure food security, resource save, and environmental protection. In maize breeding stay-green type cultivars can significantly improve grain yield, but the effects on N use efficiency remains largely unknown. Additionally, the question on whether maize grain yield and nutrient quality can be improved simultaneously by breeding new cultivars and advanced nutrient managements also remains to be addressed. Here, combining the greenhouse and field experiments, we investigated the physiological and molecular mechanism underlying how N remobilization efficiency is affected by maize stay green and is responded to various N supplies. The main findings present as follows:1) We evaluated the yield and N use efficiency of84commercial maize hybrids at three locations in North China plain in2010and2011. During last decades, stay-green breeding can improve maize grain yield and N uptake efficiency. However, over-stay-green leads to decrease of leaf N remobilization efficiency without further increasing grain yield. Thus, optimal stay-green type cultivars are important to ensure the grain yield and reduce N residue in stock.2) By the greenhouse pot experiments in two yerars,15N labeling N was used to quantitatively evaluate NUE of XY335(optimal stay-green type) and ZD958(over stay-green type). It showed that XY335has higher N remobilization efficiency (mainly in the upper and middle leaves), post-silking N uptake efficiency as well as higher photosynthetic N use efficiency than those of ZD958. Northernblot analysis further revealed that the expression of ZmGS1.4gene for N remobilization were higher in upper and middle leave of XY335than ZD958at30days after silking, suggesting a significantly higher NRE in XY335.3) We compared the expression of genes associated with the N remobilization process between hybrid XY335and NE9(over stay-green) under three N application levels in the field (45,120and240kg N ha"1). XY335showed greater yield and higher NRE at all N levels. From silking, the onset of N decrease was earlier in XY335than in NE9, and the reduction of leaf soluble protein concentration in XY335was stronger than that in NE9. Correspondingly the expression level of ZmSee2β was higher in XY335compared with that of NE9, which suggested protein degradation was more rapid in XY335.4) We applied five different N treatments, which consisted of N supplied in organic and/or inorganic forms at the following rates:0/45,0/120,0/240,120/120, and240/0kg ha-1organic/inorganic N. Yield was found to increase with increasing N input, but no significant difference was found in grain yield among0/240,240/0, and120/120treatments. Organic N application tended to decrease N accumulation and to increase N utilization efficiency. Genes related to N assimilation activity in leaves, such as ZmNR1and ZmFd-GOGATl, were unaffected by different N forms. In contrast, genes related to N remobilization activity in leaves, such as ZmGS1.1and ZmGDH1, were up-regulated, especially in old leaves, by the pure organic N supply treatment.Taken together, we conclude that performance of over stay-green in maize cultivars can not improve maize yield, but resulted in a decrease of leaf N remobilization efficiency, which hinder the coordination of grain protein content and yield. Breeding for optimal stay-green maize varieties, and also optimal N inputs can improve the remobilization of pre-silking N and contribution of post-silking N to grain. These help maize to reach high yield, great grain N harvest, and less straw N residual at the same time. |
