The study was carried out in order to explore the responses of photosynthetic performances of summer maize to nitrogen nutrition.Four treatments were designed as 0,112.5,225 and 337.5 kg/ha respectively.The results showed that P(n), chlorophyll content and soluble protein increased with the addition of nitrogen fertilization,but the degradation of chlorophyll and soluble protein delayed.Hill reaction activity and PEPCase activity of nitrogen treatments were higher than that of CK for all periods,and the mor...