Abstract: 【Objective】In order to provide a theoretical support for improving the yield and water and nitrogen use efficiency of vegetable crops in view of the questions of irrigation and nitrogen irrationally in the vegetable greenhouse for semiarid northwest area, the responsive mechanism of root growth and distribution to water and nitrogen supply was studied under the different levels of water and nitrogen treatment by the way of exploring the effects of water and nitrogen supply on crop root, yield and water and nitrogen use efficiency.【Method】 The field experiment was conducted with different N-rate and irrigation treatments. Small pumpkin “Jintong” was chosen as a test cultivar. The treatments consist of three different irrigation levels (W3, 1 500 m3•hm-2; W2, 1 100 m3•hm-2; W1, 700 m3•hm-2) and three different nitrogen rate levels (N3, 350 kg•hm-2; N2, 250 kg•hm-2; N1, 150 kg•hm-2). There were 9 treatments in total with randomized block design to study the effects of different water and nitrogen supplies on root growth, yield and water and nitrogen use efficiency of small pumpkin. 【Result】 There are 90% of total root length were concentrated in the 0-40 cm soil layer, the root length density decreased exponentially when the soil layer increased. The root length, yield, water use efficiency (WUE) increased with the improvement of N-rate, and decreased by more nitrogen application at the W1 and W2 irrigation level, but the root length increased with increase of N-rate and there were no significant difference in the yield of small pumpkin at the W3 irrigation level. Compared to N3 treatment, the root length, yield and WUE increased with the increase of irrigation amount, they decreased when the irrigation amount was more than 1 100 m3•hm-2. WUE decreased with the increase of irrigation amount and it reached the highest value (35.59 kg•m-3) in W1N2 treatment. Nitrogen use efficiency (NUE) decreased significantly with the increase of N-rate when the irrigation amount was at W2 and W3 levels, and NUE of N1 and N2 treatment increased significantly compared to N3 treatment. The order of irrigation and nitrogen fertilizer effect on total root length of small pumpkin was nitrogen, water, water and nitrogen interactions. The root length of diameter less than 2 mm presented the parabola trend when N-rate and irrigation increased, and had a better linear regression with the yield of pumpkin. A significant linear relationship between yield and root surface area and the root length of root with the diameter less than 2 mm.【Conclusion】Less or more irrigation and nitrogen fertilizer could reduce the yield, water and nitrogen use efficiency and the characteristic parameters of the roots of small pumpkin. The highest values of yield and characteristic parameters of the roots of small pumpkin were obtained when N-rate and irrigation water amount were 1 100 m3•hm-2 and 250 kg•hm-2, respectively. The water and nitrogen treatments were through the way of root length of root with the diameter less than 2 mm to improve the yield of small pumpkin. It was concluded that the optimal pattern of irrigation and nitrogen fertilization is that the irrigation water amount and N-rate are 1 100 m3•hm-2 and 250 kg•hm-2, respectively.

Key words: drip irrigation under plastic mulch , water and nitrogen supply , small pumpkin , root length density , water and nitrogen use efficiency , root growth