Chlorophyll fluorescence and photosynthetic gas exchange under direct versus diffuse light in evergreen conifer (Picea pungens) shoots and broadleaf shrub (Rhododendron ponticum) leaves

Abstract

Recent studies have shown that photosynthesis may increase under diffuse light regimes (e.g., cloud-generated) compared to direct (collimated) irradiance at the canopy and ecosystem levels. Yet, there are few comparative studies for individual species at the branch or leaf level. One of the strongest contrasts in the plant kingdom is the architectural differences between the typical conifer needle-shoot versus the broadleaf leaf habit. Thus, we investigated leaf/shoot photosynthetic gas exchange and chlorophyll fluorescence under experimental diffuse and direct light regimes for a typical conifer (Picea pungens Engelm.) and broadleaf (Rhododendron ponticum L.) species. Light-saturated photosynthesis (Asat) was 18 and 16 % greater under diffuse compared to direct light in spruce and rhododendron, respectively. Similarly, leaf conductance and transpiration were 42 and 75 %, and 11 and 21 % greater under diffuse light, respectively. The ratio of internal CO2 to ambient CO2 (Ci/Ca) was 23 % greater in the rhododendron under diffuse, saturating light, while Ci/Ca was similar in spruce. There were no significant differences in leaf chlorophyll fluorescence parameters, including the quantum yield of PSII or non-photochemical quenching for either species exposed to either direct or diffuse light. Collectively, our data suggest that diffuse compared to direct-beam light increased photosynthetic capacity at both the shoot (conifer) and leaf (broadleaf) levels, and increases were associated with enhanced stomatal opening, not differences in the photosynthetic light reactions. Importantly, the improved stomatal opening under diffuse light was not associated with any corresponding changes in environmental parameters that can strongly influence stomatal behavior, e.g., humidity or temperature.