Plant density alters nitrogen partitioning among photosynthetic components, leaf photosynthetic capacity and photosynthetic nitrogen use efficiency in field-grown cotton

Plant population density (PPD) is an important practice for optimizing canopy structure and improving canopy photosynthetic capacity of field-grown cotton (Gossypium hirsutum L.). A 2-yr field experiment was conducted to determine if and how PPD (7.5, 19.5 or 31.5 plants m−2) affects the light-saturated photosynthetic rate and photosynthetic nitrogen use efficiency in cotton leaves, with a focus on the key canopy characteristics for efficient utilization of light and nitrogen. The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by PPD. As PPD changed, cotton optimized photosynthetic N use efficiency and photosynthetic capacity by adjusting leaf mass per area, which in turn affected leaf N allocation to the photosynthetic apparatus. In the upper canopy layer, leaf N allocation to the photosynthetic apparatus increased as PPD increased, resulting in an increase in leaf photosynthetic N use efficiency. In contrast, in the mid- and lower-canopy layers, leaf N allocation to the photosynthetic apparatus decreased as PPD increased, resulting in declines in leaf light-saturated photosynthetic rate and photosynthetic N use efficiency. The overall results indicated that high photosynthetic capacity of leaves in the upper-canopy layer and high leaf N allocation to the photosynthetic apparatus and photosynthetic use efficiency of photosynthetic nitrogen in the mid- and lower-canopy layers were two key canopy characteristics for efficient utilization of light and nitrogen by cotton. The medium-PPD is the optimum plant density due to high light utilization efficiency, superior spatial distribution of leaf N allocation to the photosynthetic apparatus and photosynthetic use efficiency of photosynthetic N in leaves within the canopy.