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

item EARLES, J. MASON - Yale University
item THEROUX-RANCOURT, GUILLAUME - University Of California
item GILBERT, MATTHEW - University Of California
item McElrone, Andrew
item BRODERSEN, CRAIG - Yale University

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2017
Publication Date: 4/25/2017
Citation: Earles, J., Theroux-Rancourt, G., Gilbert, M., McElrone, A.J., Brodersen, C. 2017. Excess diffuse light absorption in upper mesophyll limits CO2 drawdown and depresses photosynthesis. Plant Physiology. 174(2):1082-1096. doi: 10.1104/pp.17.00223.

Interpretive Summary: Changes in light directionality associated with climate change, e.g. due to decreasing fog and cloud cover, will affect photosynthesis at both the leaf and canopy levels. We developed and applied a 3-D spatially-resolved gas exchange model parameterized with actual light absorption characteristics from leaves illuminated under diffuse and direct light. This demonstrated light availability can be decoupled from photosynthetic capacity, which generate an 11% decline in photosynthesis. At the canopy level, net primary productivity is higher under diffuse as compared to direct light, likely due to deeper penetration within the canopy. This study provides an important step in linking internal light absorption derived from x-ray microCT to photosynthesis, which can be used to improve predictions of how natural and agricultural vegetation will respond to future light environments. It also helps to better understand light use in agricultural canopies that vary based on trellis type (e.g. in vineyards).

Technical Abstract: Sun-grown and shade-grown leaves of some species absorb direct and diffuse light differently. Sun-grown leaves can photosynthesize ~10-15% less under diffuse compared to direct irradiance, while shade-grown leaves do not exhibit this sensitivity. In this study, we investigate if the spatial differences in light absorption in sun- and shade-grown leaves in the model species Helianthus annuus can explain their photosynthetic response to diffuse versus direct light. We used a 3-D spatially-explicit finite element gas exchange model parameterized with light absorption profiles from leaves illuminated under diffuse and direct light to demonstrate that decoupling light availability from photosynthetic capacity by changing light directionality is sufficient to generate an 11% decline in photosynthesis. This decoupling corresponds with a mismatch in leaf photosynthetic function relative to its developmental light environment, as photosynthesis of sun- but not shade-grown leaves was affected by diffuse light. Our findings imply that leaves are both anatomically and biochemically adapted to specific light environments and sensitive to the directional quality of light.