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

Area of Science:

• Plant Physiology
• Photosynthesis Research
• Agricultural Science

Background:

• Diffuse light can increase canopy productivity by penetrating deeper into foliage.
• However, individual sun-exposed leaves show reduced photosynthetic efficiency under diffuse light compared to direct light.
• The underlying mechanism for this leaf-level phenomenon remains unclear.

Purpose of the Study:

• Investigate the role of light absorption spatial distribution relative to electron transport capacity in sun- and shade-grown sunflower leaves.
• Determine if this spatial distribution explains the observed diffuse light photosynthetic depression.
• Explore the impact of diffuse light on photosynthetic efficiency in different light environments.

Main Methods:

• Utilized a one-dimensional porous medium finite element gas-exchange model.
• Parameterized the model with light absorption profiles from sun- and shade-grown sunflower leaves.
• Analyzed the spatial distribution of light absorption and its relationship with electron transport capacity.

Main Results:

• In sun-grown leaves, diffuse light led to less penetration into the mesophyll compared to direct light.
• This resulted in heterogeneous saturation of electron transport capacity and reduced CO2 drawdown.
• A significant decline (11%) in photosynthesis was observed in sun-grown leaves under diffuse light, but not in shade-grown leaves.

Conclusions:

• The spatial distribution of light absorption relative to electron transport capacity explains diffuse light photosynthetic depression in sun-grown leaves.
• Shade-grown leaves, being thinner, distribute light more evenly, mitigating this effect.
• This phenomenon can influence canopy-level productivity, especially in canopies with low light extinction coefficients or limited leaf area.