Increased conductance associated with higher photosynthesis in leaves and green stems of avocado exposed to diffuse light

Abstract

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.