Optimal water and nitrogen management increases cotton yield through improving leaf number and canopy light environment

Recently-published information on cotton agronomic and light energy use responses to nitrogen (N) under varying water availability is limited in Xinjiang, northwest China. A two-year field experiment was conducted to evaluate the effects of two irrigation patterns (I400, conventional irrigation; I267, limited irrigation) and three N fertilizer rates (N320, 320; N272, 272; N224, 224 kg N ha−1) on leaf number (LN), mean inclination angle (MIA) of leaves, light interception rate (LIR), dry matter accumulation, and seed cotton yield. Under the I400 condition, seed cotton yield for N272 was comparable to that for N320, but 9.6% higher than that for N224. Compared with N320, N272 plots exhibited a 4.7% reduction in biological yield and 4.9% increase in harvest index. N272I400 slightly reduced the leaf number and increased the MIA of the top-canopy, and improved the LN and LIR in the middle canopy, canopy photosynthetic rate (CAP), and dry matter accumulation. In contrast, under I267 conditions, N320 significantly increased leaf area and achieved the better LIR, dry matter accumulation, and yield than N272 and N224. The LIR induced by the variation in leaf number was strongly associated with CAP and dry matter accumulation. The LN of the middle canopy likely represented the strong link in photosynthetic response to N rate and water supply. Leaf number was strongly associated with leaf area and was negatively correlated with MIA. These results suggest that leaf number is a key regulatory factor for regulating canopy leaf inclination and light interception. We suggest that estimates of light energy utilization should be included along with leaf number estimates in future efforts to scale from leaf physiology to canopy construction in cotton.