Effects of water and nitrogen regulation on physiological characteristics and yield of cotton

Water and nitrogen are the main factors affecting the growth, development, and yield of cotton. In this study, “Nongda Cotton No. 36” was selected to investigate the effects of water and nitrogen on morphology, physiological characteristics, and yield of cotton. Two water conditions were set: drought stress (W1, relative water content was 45%±5%) and normal water supply (W2, relative water content was 70%±5%), and three nitrogen levels: no nitrogen (N0), low nitrogen N1, 69 mg(N)∙kg⁻¹, and normal nitrogen fertilizers N2, 138 mg(N)∙kg⁻¹. Changes in aboveground and root morphology, photosynthetic characteristics, antioxidant enzyme activity, nitrogen metabolism enzyme activity, and cotton yield were analyzed under different water and nitrogen fertilizer conditions. The results showed that compared with the W2 treatments, the W1 treatments significantly inhibited cotton growth and decreased plant height, stem diameter, leaf area, total root length, total root surface area, and average root diameter (P<0.05). The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were significantly enhanced, the relative chlorophyll content (SPAD) and maximum photochemical efficiency (F_v/F_m) were decreased, and the photosynthetic capacity was weakened, resulting in a decrease in cotton yield (P<0.05). Compared with the N2 treatments, the N0 and N1 treatments significantly reduced plant height, stem diameter, leaf area, total root length, total root surface area, and average root diameter of cotton (P<0.05). The activities of SOD, POD, CAT, glutamine synthetase (GS), nitrate reductase (NR), SPAD, and F_v/F_m in cotton were significantly decreased under the N0 and N1 treatments, and the photosynthetic capacity of cotton was weakened. Thus, the cotton yield decreased (P<0.05). Under drought stress, conventional nitrogen application promoted the growth of the aboveground and underground parts of cotton; significantly increased the SPAD, net photosynthetic rate, and F_v/F_m in main-stem leaves (P<0.05); and enhanced the activities of antioxidant enzymes (SOD, POD, and CAT) and nitrogen metabolism enzymes (GS and NR) (P<0.05), which alleviated the damage caused by drought stress and increased the cotton yield. Under low-nitrogen conditions, the normal water supply treatment promoted the growth of cotton; enhanced photosynthesis, nitrogen metabolism enzyme activities, and yield (P<0.05); and alleviated the adverse effects of low-nitrogen stress on cotton. Therefore, cotton yield under drought stress can be increased by increasing nitrogen fertilizer, and cotton yield under low-nitrogen stress can be increased by appropriately increasing irrigation water. The results provide a theoretical basis for clarifying rational water and fertilizer management of cotton under nitrogen and water stresses.