Natural light illuminating a leaf has both diffuse and direct components. Under clouds or a dense canopy, virtually all the light is diffuse, but even on a sunny day about fifteen percent of the light a leaf “sees” is diffuse. Moreover, the ratio of diffuse to direct light is increasing globally, as atmospheric particulates increase with pollution and as atmospheric water vapor increases with global warming. Effects of direct and diffuse light on photosynthesis vary with level of organization. Diffuse light increases photosynthesis at the canopy level because diffuse lights penetrates better to the lower leaves. However, leaves have optical features such as columnar palisade cells that favor penetration of direct light, and at the leaf level direct light is more effective at driving photosynthesis than diffuse light. To explore this leaf-level difference, we used PAM measurements of chlorophyll fluorescence from Antirrhinum leaves exposed to direct and diffuse light.

Results/Conclusions

Induction curves, which measure changes in yield following a dark-to-light transition, display a greater decrease in yield and slower recovery when the light is diffuse than when it is direct, even when irradiance (1000 µmol m^-2 s^-1) and spectral quality are identical. Light curves show a greater decrease in yield with increasing irradiance when light is diffuse than when it is direct. These results suggest that the cells generating the fluorescence signal—primarily those close to the surface of the leaf—downregulate photosynthetic electron transport more under diffuse than under direct light.