Effects of coupling water and nitrogen on root physio-ecological indices and yield of adzuki bean

The process of water regulation during the growing period of adzuki bean under experimental pot conditions was studied in order to determine the effects of combining different water (drought stress and normal irrigation) and nitrogen fertilizer (N application rates of 0 g·kg^-1, 0.1 g·kg^-1, and 0.3 g·kg^-1) conditions on the root physio-ecological indices and yield. The results showed that: 1) Under drought stress condition, plant height, stem diameter, shoot dry weight, total root length, root area, root volume, average root diameter, max root length, root dry weight and seedling index initially increased and then decreased with increasing nitrogen application rate. Also root soluble protein content at seedling stage, P_n, T_r and G_s at blooming/pod-setting stage, transverse diameter, weight per pod and seeds per pod, 100-seed weight at mature stage initially increased and then decreased with increasing nitrogen application rate. Furthermore, the activities of SOD and POD in root increased with increasing nitrogen rate while soluble sugar content decreased initially and then increased with increasing nitrogen application rate. However, root MDA content, pod length and pods per plant did not significantly change in response to different nitrogen fertilizer treatments. Under normal irrigation, with increasing nitrogen application rate, maximum root length decreased. Also while SOD activity, POD activity and Gs increased, MDA content and soluble sugar content decreased initially and then increased with increasing nitrogen application. Then pods per plant did not change for three nitrogen treatments while the other indices mentioned above changed parabolically with increasing nitrogen application rate. 2) Under equal nitrogen fertilizer treatments, normal irrigation decreased SOD and POD activities, soluble sugar content and soluble protein content as opposed to drought stress treatment. However, MDA content, transverse diameter and pods per plant had no significant difference for both drought and normal irrigation conditions. The other above indices increased under normal irrigation treatment compared with drought stress treatment. 3) Among the three nitrogen levels, yield of adzuki bean was highest under 0.1 g·kg^-1 nitrogen application. It increased respectively by 95.2% and 118.3% compared with 0 g·kg^-1 and 0.3 g·kg^-1 nitrogen treatments under drought stress condition. It also increased respectively by 63.8% and 137.1% compared with 0 g·kg^-1 and 0.3 g·kg^-1 nitrogen treatments under normal irrigation condition. In addition, yield of adzuki bean was higher in normal irrigation treatment. It increased by 84.5% and 198.7%, respectively, compared to drought stress treatment under equally nitrogen fertilizer treatment. The results suggested that reasonable application of nitrogen fertilizer and water enhanced the growth and yield of adzuki bean, furthermore, adzuki bean was more suitable for production at low soil moisture (35%–45% of field water capacity) and suitable fertilizer condition (0.1 g·kg^-1 N application) in the hilly area of Loess Plateau of Shanxi Province.