Effects of water-nitrogen coupling on photosynthetic characteristics, yield, water and nitrogen use efficiency for mountain apple trees under surge-root irrigation in Northern Shaanxi area of China.

Taking 7-year-old apple trees (Hanfu) as the test material, an experiment with three irrigation levels including high water (W, 85%-100%, was the field water holding capacity), medium water (W, 70%-85%) and low water (W, 55%-70%), and three nitrogen application levels, high (N, 600 kg·hm), medium (N, 400 kg·hm) and low (N, 200 kg·hm), was conducted to investigate the effects of water and nitrogen coupling on photosynthetic characteristics, yield and water and nitrogen utilization of apple trees in mountainous areas under surge-root irrigation (SRI). The results showed that the net photosynthetic rate (), transpiration rate (), stomatal conductance (), intercellular CO concentration (), leaf instantaneous water use efficiency (WUE) of apple trees leaves decreased with decreasing nitrogen application rates under the same irrigation amount, but increased. Under the same nitrogen application rate, foliar , , and WUE decreased with decreasing irrigation amount, but increased. The daily average values of and under WN treatment were the largest, while WN treatment had the largest WUE. Apple yield, irrigation water use efficiency (IWUE) and nitrogen partial productivity (NPFP) were significantly affected by irrigation and nitrogen application. The WN treatment had the highest yield (26761 kg·hm). IWUE increased significantly with the decreasing irrigation and the increasing nitrogen application, while NPFP increased significantly with the increases of irrigation and the decreases of nitrogen application. Results of the regression analysis showed that the combination of irrigation and nitrogen application was closest to WN treatment when yield and IWUE got the optimal solution. Therefore, WN treatment was the best combination mode of water and nitrogen application for apple under SRI in Northern Shaanxi mountain area.