Enhanced coordination of photosynthetic functions among cotton boll–leaf systems to maintain boll weight under high-density planting

High planting density curtails the boll number per plant more significantly than the single boll weight, yet it is hard to estimate the boll weight from single-leaf photosynthesis with increasing boll abscission. We speculated that high plant density may lead to coordination among photosynthetic organs to maintain boll weight. Therefore, cotton (Gossypium hirsutum L.) yield formation, the photosynthetic characteristics of the leaves and boll–leaf system were studied under various plant densities. The results showed that the boll number per plant or boll number per boll–leaf system decreased more greatly than the boll–leaf system number per plant with increasing plant density. Leaf area, single leaf photosynthetic rate, and CO₂ assimilation of the boll–leaf system all gradually decreased with the increase of plant density. There was a significant positive linear correlation between integrated CO₂ assimilation of the boll–leaf system and boll biomass per boll–leaf system. After girdling treatment, the boll biomass of the boll–leaf system decreased more greatly compared with non-girdling treatment with increasing plant density. Moreover, the girdling/non-girdling of boll biomass per boll–leaf system reached 0.8–1.0 at 19–25 plants m⁻². The removal of the lower-canopy bolls caused a significant increase in the boll biomass of the upper canopy, and the biomass per boll at high densities (>25 plants m⁻²) increased more greatly than at low densities. Therefore, the rapid decrease in CO₂ assimilation of the boll–leaf system resulted in a decreased boll number per boll–leaf system as plant density increased (<25 plants m⁻²). Under high densities (>25 plants m⁻²), the boll biomass not only depends on the photosynthetic rate of the corresponding boll–leaf system, but also on the coordination of photosynthetic functions among adjacent cotton boll–leaf systems. Optimal planting density (19–25 plants m⁻²) means that the assimilate production and utilization of the boll–leaf system can be balanced. At this density, the coordination of boll number and boll weight is conducive to maximizing the yield per plant and unit ground area.