While considerable attention has been paid to how plants respond to changes in the spectral distribution and quantity of light, less attention has been paid to how plants respond to changes in the angular qualities of light. Evidence from both leaf- and ecosystem-scale measurements indicate that plants vary in their response to diffuse compared to direct light growing environments. Because of the significant implications for agricultural production, we quantified how changes in light quality affect the structure, function, and growth of Roma tomatoes in an open-air greenhouse experiment with direct and diffuse light treatments. Diffuse light conditions (ca. 50-60% diffuse) were created with a glass coating to diffuse light without significantly reducing the quantity of light. We measured leaf physiology and structure, as well as whole plant physiology, morphology, and growth. Light-saturated photosynthetic rates were set by the growing light environment and were unchanged by short-term exposure to the opposite light environment. Thus, after two months, plants in the diffuse light treatment demonstrated lower photosynthesis and had thinner leaves with higher chlorophyll concentration. However, relative growth rates did not differ between treatments and plants grown in diffuse light had significantly higher biomass at the conclusion of the experiment. While there was no difference in leaf or whole-plant water-use efficiency, plants in the diffuse light treatment demonstrated significantly lower leaf temperatures, highlighting the potential for diffuse light coatings and/or materials to reduce greenhouse energy use. Our results highlight the need to advance our understanding of the effects of diffuse light conditions on agricultural crops growing on a changing planet.
Research has demonstrated that diffuse light drives changes in leaf photosynthesis, with the direction and magnitude varying across species; however, our understanding of the relationship between diffuse light and plant gas exchange, as well as the mechanisms driving these relationships remain unresolved. We studied the effects of diffuse light on plant function in potted individuals of Persea americana (avocado). We first measured leaf gas exchange subject to varying proportions of direct and diffuse light, as well as photosynthetic response to varying CO2 (A-Ci curves) in predominantly direct and predominantly diffuse light. We find that leaf photosynthetic rates increase as the proportion of diffuse light increases and that those changes are associated with stomatal conductance, rather than photosynthetic biochemistry. Given that avocados have green stems, we then measured stem gas exchange in predominantly direct compared to predominantly diffuse light. While we also observed an increase in conductance in stems subject to diffuse light, there was not an increase in photosynthetic rate, effectively decoupling gas flux from carbon gain. Finally, by scaling measurements of gas exchange to the plant, we demonstrate that stem bark conductance contributes proportionally more to whole-plant conductance under diffuse light. Our results add to our understanding of the potential mechanisms that govern how plant function varies in response to changes in light quality, the first paper to demonstrate mechanisms to explain increases under diffuse light. As diffuse light increases globally, this variable needs to be integrated into our understanding of plant carbon-water tradeoffs in response to climate change.
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