Cotton responds to different plant population densities by adjusting specific leaf area to optimize canopy photosynthetic use efficiency of light and nitrogen

•Cotton adjusts its specific leaf area in response to the spatial distribution of light within the canopy.•Cotton optimizes the canopy use efficiency of N and light by changing its specific leaf area.•Optimization of canopy photosynthetic capacity in the mid- and lower-canopy layer is key to maximizing whole-canopy photosynthesis. Appropriate plant population density (PPD) is an important crop management practice for optimizing canopy light distribution and increasing canopy photosynthetic capacity in field-grown cotton (Gossypium hirsutum L.). A 2-year field experiment was conducted to determine how the PPD (7.5, 19.5 or 31.5 plantsm−2) of cotton affects canopy photosynthetic capacity and photosynthetic use efficiency of light and N. The results indicated that PPD significantly affected both leaf morphology and canopy photosynthetic characteristics. As PPD changed, cotton maximized canopy apparent photosynthetic light use efficiency (CAPLUE) and photosynthetic capacity by adjusting specific leaf area (SLA), which in turn affected leaf N distribution in the canopy. The SLA of all three canopy layers increased as PPD increased. In the upper canopy, canopy light interception and canopy apparent photosynthetic N use efficiency (CAPNUE) rose as SLA increased, but CAPLUE declined. As PPD increased, SLA in the mid- and lower-canopy layers increased significantly. This caused canopy apparent photosynthesis rate per leaf area (CAPLeaf) and CAPNUE to decline. Medium-PPD had the highest canopy apparent photosynthesis rate (CAP) and CAPLUE in the mid- and lower-canopy layers. As a result, medium-PPD had the highest whole-canopy photosynthetic capacity and CAPLUE in this study. Overall, the results indicated that optimal spatial distribution of both light and specific leaf area is key for efficient utilization of light and N in cotton canopies.