Differential Photosynthetic Acclimation to Fluctuating Light Reveals Specific Photoprotective Strategies in...

In intercropping systems, understory crops like soybean (Glycine max (L.) Merr.) experience fluctuating light (FL) environments characterized by morning and afternoon light limitation and midday overexposure. To investigate photosynthetic acclimation to such FL conditions, we compared the shade-tolerant cultivar ND12 and the shade-sensitive cultivar GX7. Plants were grown under either constant light (CL) or FL, both with equivalent average light intensity. We comprehensively assessed the effects of FL on plant growth, leaf structure, and photosynthetic performance. Results demonstrated that: (1) FL significantly reduced leaf area, biomass accumulation, and net photosynthetic rate (P_n), with the magnitude of these effects modulated by cultivar-specific shade tolerance through changes in leaf anatomy and stomatal traits. (2) FL enhanced the activity of photosystem II (PSII) in soybeans but inhibited photosystem I (PSI), while simultaneously inducing significant activation of non-photochemical quenching (NPQ), thereby contributing to photoprotection. (3) ND12 outperformed GX7 in maintaining PSII functionality and photoprotective capacity under FL, while demonstrating superior light-use efficiency with lower metabolic costs. (4) Proteomic data also showed that photosynthesis was mainly affected by the cultivar's shade tolerance under FL. Consequently, this study proposes that NPQ modulation represents an essential adaptive strategy for maintaining soybean photosynthesis under FL, with cultivar shade tolerance emerging as the key regulatory factor.