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Excess diffuse light absorption in upper mesophyll limits CO2 drawdown and depresses photosynthesis.

Excess diffuse light absorption in upper mesophyll limits CO2 drawdown and depresses photosynthesis.
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Earles JM, Théroux-Rancourt G, Gilbert ME, McElrone AJ, Brodersen C,


Earles JM, Théroux-Rancourt G, Gilbert ME, McElrone AJ, Brodersen C, (click to view)

Earles JM, Théroux-Rancourt G, Gilbert ME, McElrone AJ, Brodersen C,

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Plant physiology 2017 04 21() pii pp.00223.2017
Abstract

In agricultural and natural systems, diffuse light can enhance plant primary productivity due to deeper penetration into and greater illumination of the entire canopy. However, for individual sun-grown leaves from three species, photosynthesis is actually less efficient under diffuse compared to direct light. Despite its potential impact on canopy-level productivity, the mechanism for this leaf-level ‘diffuse light photosynthetic depression’ effect is unknown. Here we investigate if the spatial distribution of light absorption relative to electron transport capacity in sun- and shade-grown H. annuus leaves underlie its previously observed diffuse light photosynthetic depression. Using a new one-dimensional (1-D) porous media finite element gas exchange model parameterized with light absorption profiles, we found that weaker penetration of diffuse versus direct light into the mesophyll of sun-grown H. annuus leaves led to a more heterogeneous saturation of electron transport capacity and lowered its [CO2] draw-down capacity in the intercellular airspace and chloroplast stroma. This decoupling of light availability from photosynthetic capacity under diffuse light is sufficient to generate an 11% decline in photosynthesis in sun- but not shade-grown leaves, primarily because thin shade-grown leaves similarly distribute diffuse and direct light throughout the mesophyll. Finally, we illustrate how diffuse light photosynthetic depression could overcome enhancement in canopies with low light extinction coefficients and/or leaf area, pointing toward a novel direction for future research.

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