An elevated-CO₂ world is likely to have a greater proportion of diffuse light, because of increased atmospheric concentrations of H₂O. Diffuse light penetrates canopies better than direct light and causes higher canopy-level photosynthetic rates.  By contrast, direct light causes higher rates of photosynthesis at the individual-leaf level. This might be caused simply by increased absorptance of direct vs. diffuse light.  In order to evaluate this possibility, we constructed an apparatus for measuring leaf absorptance in both diffuse and direct light. In a previous study, leaf discs stored in the dark did show slightly higher absorptance of direct than diffuse light, but this difference was insufficient to account for their higher photosynthetic rates. However, leaves in nature (and in gas-exchange cuvettes) are exposed to direct or diffuse light for long periods, so we examined how pre-exposure to direct light, diffuse light, or darkness affected leaf absorptance.

Results/Conclusions

Pre-exposure had little or no effect on thick leaves (Helianthus, Antirrhinum) but in thin-leafed Alocasia, pre-exposure to direct light significantly increased transmittance especially when also measured in direct light.  This is consistent with movement of chloroplasts into the anticlinal position. The absorptance changes resulting from this are small and are still insufficient to explain increased photosynthesis under direct light.  We hypothesize that the enhanced rate of photosynthesis observed under direct light is caused by greater penetration of light into the leaf interior.