A new paradigm in leaf‐level photosynthesis: direct and diffuse lights are not equal

Abstract

Global-change scenarios suggest a trend of increasing diffuse light due to expected increases in cloud cover. Canopy-level measurements of plant-community photosynthesis under diffuse light show increased productivity attributed to more uniform distribution of light within the forest canopy, yet the effect of the directional quality of light at the leaf level is unknown. Here we show that leaflevel photosynthesis in sun leaves of both C3 and C4 plants can be 10-15% higher under direct light compared to equivalent absorbed irradiances of diffuse light. High-lightgrown leaves showed significant photosynthetic enhancement in direct light, while shade-adapted leaves showed no preference for direct or diffuse light at any irradiance. Highlight-grown leaves with multiple palisade layers may be adapted to better utilize direct than diffuse light, while shade leaf structure does not appear to discriminate light based on its directionality. Based upon our measurements, it appears that leaf-level and canopy-level photosynthetic processes react differently to the directionality of light, and previously observed increases in canopy-level photosynthesis occur even though leaf-level photosynthesis decreases under diffuse light.

Excess Diffuse Light Absorption in Upper Mesophyll Limits CO2 Drawdown and Depresses Photosynthesis

“…However, the thin shade leaf approached its photosynthetic capacity beyond 750 mmol m 22 s 21 PPFD. Previously observed data by Brodersen et al (2008) are also shown in Figure 5 for comparison.…”

“…While the reasons for this discrepancy are unclear, it is possible that the sun-and shade-grown leaves differentially incurred photoinhibition due to excess light absorption during the light acclimation period. Specifically, Brodersen et al (2008) acclimated both sets of leaves at 500 mmol m 22 s 21 , which would likely induce different degrees of photoinhibition for the sun-grown versus shade-grown leaf. Without spatially resolved data on the degree of nonphotochemical quenching throughout each leaf, however, we cannot be certain of the mechanism underlying these apparent differences in light-limited f PSII between sun-grown and shade-grown leaves.…”

“…Specifically, Brodersen et al (2008) attributed these photosynthetic differences to deeper penetration of direct compared with diffuse light. That is, when light enters the leaf, it is absorbed or scattered, thereby establishing light gradients within the mesophyll.…”

“…Moreover, by directly coupling light absorption to CO 2 consumption, we predicted CO 2 concentrations [CO 2 ] within the intercellular airspace and chloroplast stroma. For a given [CO 2 ] distribution, we calculated leaf-level photosynthetic output and compared it with previously measured values by Brodersen et al (2008). We then tested the photosynthetic sensitivity of sun-and shade-grown leaves to select anatomical and biochemical parameters ( Table II).…”

Regional crop yield, water consumption and water use efficiency and their responses to climate change in the North China Plain

“…Canopy-level measurements of photosynthesis under diffuse light show increased productivity attributed to more uniform distribution of light within a forest canopy. Brodersen et al (2008) show that leaf-level and canopy-level photosynthetic processes react differently to the directionality of light, and increases in canopy-level photosynthesis occur even though leaf-level photosynthesis decreases under diffuse light. It is important to study regional haze in China on climate and crop production.…”