Increased photosynthesis from a deep-shade to high-light regime occurs by enhanced CO2 diffusion into the leaf of Selaginella martensii

The current understanding of photosynthesis across land plant phylogeny strongly indicates that ancient vascular plants are mainly limited by strong constitutive CO2 diffusional constraints, particularly low stomatal and mesophyll conductance. Considering that the lycophyte Selaginella martensii can demonstrate long-term light acclimation, this study addresses the regulation extent of CO2 assimilation in this species cultivated under contrasting light regimes of deep shade, medium shade and high light. Comparative analyses of photosynthetic traits, CO2 conductance and leaf morpho-anatomy revealed acclimation plasticity similar to that of seed plants, though occurring in the context of an inherently low photosynthetic capacity typical of lycophytes. Specific modulations of the stomatal density and aperture, chloroplast surface exposed to mesophyll airspaces and cell wall thickness sustained a marked improvement in CO2 diffusion from deep shade to high light. However, the maximum carboxylation rate was comparatively less effectively upregulated, leading to a greater incidence of biochemical limitations of photosynthesis. Because of a low carboxylation capacity under any light regime, a lycophyte prevents potential photodamage to the chloroplast by not only exploiting the thermal dissipation of excess absorbed energy but also diverting a large fraction of photosynthetic electrons to sinks alternative to carboxylation.