Photosynthetic mechanism of cotton under fluctuating light field planted with different densities

Leaves, especially in the lower canopy, experience a complex and variable light condition that determines photosynthesis and biomass accumulation. However, the photosynthetic mechanism under fluctuating light filed remains controversial. We investigated leaf morphology, stomatal anatomy, gas exchange, chlorophyll fluorescence, proteomics, biomass, and yield in one field experiment (three plant densities of cotton) and two simulation experiments (light fluctuation frequency and intensity). Field experiment indicated that higher plant density significantly reduced light intensity but increased the frequency of light fluctuation in lower canopy, and this exerted a detrimental effect on leaf photosynthesis, biomass accumulation and yield. The simulation experiments revealed that increase in light intensity led to higher photosynthetic rate ( A N ) and biomass supported by enhancing leaf area, chlorophyll content, maximum photochemical efficiency, and stomatal conductance and density. Under the same light intensity, higher frequency of light fluctuation had no impact on A N and biomass owing to consistent trait variations regarding light harvesting and conversion, and carbon gain. Proteomic data also demonstrated that photosynthetic capacity was mainly determined by light intensity under fluctuating light field. Consequently, this study emphasizes an appropriate improvement of light intensity plays a crucial role in enhancing photosynthetic productivity and cotton yield under fluctuating light field planted with different densities. • High plant density significantly reduced light intensity but increased the frequency of light fluctuation in low canopy. • Increasing light intensity but not fluctuation frequency can largely enhance photosynthesis and biomass.