More shade or sun? How fluctuating light affects photosynthesis in cotton.

Natural light conditions are inherently dynamic, yet their effects on plant light acclimation remain ambiguous. We established two light regimes with the same daily quantum input [constant light (CL) and fluctuating light (FL)] with cotton (Gossypium hirsutum L.) cultivar Xinkang4 as the object, and measured leaf anatomy, chlorophyll fluorescence, photosynthetic gas exchange, and light induction kinetics. (1) Compared with CL, FL induced more shade-acclimated leaf morphology, with total leaf thickness reduced by 8.2%, palisade layer thickness by 11.3%, and total stomatal density by 16.7%. (2) Compared with CL, FL induced more sun-acclimated photochemistry, evidenced by 12.0% lower chlorophyll content, 29.6%-32.3% higher PSI (and CEF) electron transport rates, 24.0%-28.2% improved photochemical quantum yield, and 42.3% higher carboxylation capacity, collectively leading to an increase in maximum photosynthesis of 11.8%. (3) Light induction kinetics showed that despite superior photosynthesis rates under FL, slower rate of stomatal responses to light fluctuations constrained CO2 uptake, negating the differences in photoassimilate accumulation between treatments. This study demonstrates the unique structure-function decoupling strategy for light acclimation in the cotton cultivar Xinkang4, providing a novel framework for improving crop light management.