Optimal planting density enhances cotton yield by coordinating boll–leaf system photosynthesis under heat-limited conditions

Abstract

Context

In Xinjiang, China, a region characterized by abundant solar radiation but limited heat resources, high-density cotton planting has driven remarkable yield gains, yet the underlying physiological mechanisms remain poorly understood. We hypothesized that optimal planting density enhances radiation use efficiency (RUE) and heat use efficiency (HUE) by regulating the photosynthetic performance of the boll–leaf system (a functional unit comprising a fruiting boll, its subtending main-stem leaf, and associated sympodial leaves) through optimization of canopy structure and microenvironment.

Methods

We conducted a six-year field experiment to compare four planting densities (2.9, 12.6, 24.5, and 34.6 plants m⁻²) in Xingjiang. The boll–leaf system photosynthesis, canopy microenvironment including vapor pressure deficit (VPD) and photosynthetically active radiation, as well as dry matter accumulation, lint yield, RUE, and HUE were examined.

Results

Our results demonstrated that 24.5 plants m⁻² maximized lint yield (mean 2786 kg ha⁻¹) and RUE (2.72 g per megajoule). This optimal density increased boll–leaf systems per unit area to 186–202 m⁻² (4.3-fold higher than the lowest density) while limiting the decline in boll–leaf system photosynthetic rate to 27 % (versus 52 % at 34.6 plants m⁻²). Critically, it improved canopy microclimates by reducing VPD by 14–27 % and lowering maximum temperatures by 0.5–1.7°C in middle and lower canopy layers, thereby extending daily photosynthetic duration. These microenvironmental modifications drove a 27.1 % improvement in RUE relative to suboptimal densities and maximized HUE (1.53 g per degree Celsius per day per square meter).

Conclusions

We conclude that optimal planting density achieves high yields in heat-limited regions by coordinating two key properties of the boll–leaf system: increasing the number of photosynthetic units while minimizing declines in their individual efficiency. This coordination is mediated through canopy structural adjustments that alleviate high-temperature and VPD stress, providing a physiological framework for optimizing cotton cultivation in thermal deficit environments globally.