| Drought is very important stress factor to plant growth and development, and it has become the limiting factor of agricultural development in many areas. So it is very necessary to improve the drought resistance of plant in agricultural production and environmental protection. Fertilizer application is the key technique of agricultural production, and the N fertilizer is useful for drought resistance. So it is an effective way in reducing the Water Stress by rational application of N fertilizer. The purpose of this study is to explore the drought resistance of N fertilizer in continuous drought by analyzing effects of N on growth of summer maize under different drought degrees, threshold response of drought and threshold of drought.A field experiment was conducted under rainproof sheds. There were seven drought treatments in which soil drying lasted 0 to 32 days and three N rates including N 0, 140, 280 kg/hm2 in this experiment. The summer maize was sowed in the June 28, 2006, and the drought treatment Continued from July 18, 2006 to august 22, 2006. Summer maize morphological changes, including plant height, leaf blade length, leaf blade width and stem diameter were observed during the soil drying; the canopy temperature of summer maize was observed by Thermometer STProPlus at 10:00, 12:00 and 14:00 of fine day, Meanwhile, the air temperature and air relative humidity were observed; The soil moisture content was explored by MiniTrase TDR, which made by the company santa Barbara of America. In the end, the biomass of summer maize was measured after harvest. The main findings are as follows:The effects of N rates on summer maize differed with different drought degrees. Summer maize morphological changes, including plant height, leaf blade length, leaf blade width, stem diameter, biomass and yield, increased with the amount of N fertilizer applied under slight drought condition. Under moderate soil drought, medium amount of N was better than zero N or high level of N in increasing morphological changes, biomass and yield of summer maize. Plant height, leaf blade length, leaf blade width, stem diameter, biomass and yield decreased when N amount increased under serious soil drought condition.The threshold response occurred respectively 21, 20 and 19 days after drought treatment on N rates of NO, N1 and N2, and the drought index D were 0.4164, 0.4360 and 0.5209 when threshold response occurred. Therefore, the drought stress degree will increase with the N amount increase under serious soil drought condition.There were prominent negatively liner relationship between Tc-Ta and VPD under adequate water supply. The CWSI decreased with the amount of N fertilizer applied under slight drought condition, and the CWSI increased with the amount of N fertilizer applied under serious soil drought condition. Meanwhile, this study showed that there were also prominent negatively liner relationship between yield of summer maize and CWSI under three N rates and the threshold of CWSI were found by these liner equations. The threshold of CWSI in N rates of NO, N1 and N2 were 0.115, 0.113 and 0.111, and the critical yield were respectively 6034kg·hm-2, 6257kg·hm-2, and 5991kg·hm-2.This study showed that, there was obvious interaction of N rates and soil moisture, and proper N amount should be chosen in different soil moisture condition. N fertilizer can make the threshold response take place in advance, and N fertilizer can also increase the water stress degree when threshold response occurred. But it doesn't means than N fertilizer is harm for corps which under the drought condition. It tells us that proper N amount should be chosen in different drought degrees. Before the threshold response occurred, namely the summer maize was in the slight drought or medium drought proper N amount should be chosen so that the harm of drought can be reduced. Sometimes application of N fertilizer can increase yield of Crops, and that maybe the best effect of the interaction of soil moisture and N rates.
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