Effects of nitrogen and potassium on photosynthetic characteristics in summer maize leaves under long-term water stress

Two cultivars of summer maize (Zea mays L.) with known drought-resistance rankings (Shaandan 9 more tolerant than Shaandan 911) were subjected to progressive soil water stress in pot throughout their growth period to investigate effects of nitrogen and potassium on net photosynthetic rate (Pn), intercellular CO2 concentration (Ci) and transpiration rate (Tr) in maize leaves at different growing stages for revealing their biological function in increase of plant resistance to drought in view of photosynthetic metabolism. Results showed that under a long-term water stress condition, net photosynthetic rate (Pn), Intercellular CO2 concentration (Ci) (except mature stage) and transpiration rate (Tr) in leaves were significantly decreased as well as chlorophyll content for both the drought resistant and non-drought resistant varieties. And the decline rates of them were more obvious for the non-drought resistant variety than the drought resistant one. Moreover, the chlorophyll content and Pn were higher while Tr and Ci lower for the drought resistant variety compared with the drought sensitive one .The lower Pn and chlorophyll content were recorded at seedling stage than those at elongation and heading stages, which showed that the two varieties responses to water stress were weaker at earlier than later stages. Their peak values occurred at heading stage. Application of nitrogen raised Pn and chlorophyll content whereas decreased Tr and Ci at different extent and thus alleviated the detrimental effects of water stress on photosynthesis of plants. Under the water-stressed condition, addition of N fertilizer significantly increased Pn and chlorophyll content while decreased Tr and Ci for the non-drought resistant variety. And a significant difference of them was obtained among N rates. The significant response of drought resistant variety to N addition was merely found at low N rate and there was no significant difference between high and low N rates. K exhibited more significant biological function than N in improvement of photosynthesis at the same level water stress. However, with adequate water supply, effects of N and K fertilization were significantly decreased. All the results indicated that addition of optimal nitrogen and potassium had made an efficient contribution to enhancing drought-resistance ability of a crop by improvement of its photosynthetic traits.