High photosynthesis rates in Brassiceae species are mediated by leaf anatomy enabling high biochemical capacity, rapid CO₂ diffusion and efficient light use

Certain species in the Brassicaceae family exhibit high photosynthesis rates, potentially providing a valuable route toward improving agricultural productivity. However, factors contributing to their high photosynthesis rates are still unknown. We compared Hirschfeldia incana, Brassica nigra, Brassica rapa and Arabidopsis thaliana, grown under two contrasting light intensities. Hirschfeldia incana matched B. nigra and B. rapa in achieving very high photosynthesis rates under high growth-light condition, outperforming A. thaliana. Photosynthesis was relatively more limited by maximum photosynthesis capacity in H. incana and B. rapa and by mesophyll conductance in A. thaliana and B. nigra. Leaf traits such as greater exposed mesophyll specific surface enabled by thicker leaf or high-density small palisade cells contributed to the variation in mesophyll conductance among the species. The species exhibited contrasting leaf construction strategies and acclimation responses to low light intensity. High-light plants distributed Chl deeper in leaf tissue, ensuring even distribution of photosynthesis capacity, unlike low-light plants. Leaf anatomy of H. incana, B. nigra and B. rapa facilitated effective CO₂ diffusion, efficient light use and provided ample volume for their high maximum photosynthetic capacity, indicating that a combination of adaptations is required to increase CO₂-assimilation rates in plants.