A few years ago, I was writing an application to come and join the EcoFun team in the middle of a gas-exchange measurement campaign. The trees had flushed right before we started the campaign and on the first day of our campaign, we happened to clamp onto some not fully developed leaves. When I looked at the data, I noticed that the rates of carbon and uptake and stomatal conductance of mature and developing leaves were not necessary comparable and I was intrigued. I run my doubts by Belinda Medlyn, but at that time, we didn’t have a large enough dataset to tease apart what was going on and we told ourselves that this was worth exploring, so it went onto the application for my next project.
Finally, when I joined the EcoFun team, one of the first things we did was set a glasshouse experiment that would allow us to specifically test whether stomatal behavior was conserved or not along leaf ontogeny in multiple plant functional types. For our experiment, we chose species representative of six plant functional types, of economic and ecological relevance for the local ecosystems in the South-West of France. We measured the coupling of stomatal water loss and carbon again across a gradient of atmospheric drought (vapor pressure deficit) in mature and developing leaves and under two contrasting watering regimens. This dataset allowed us to compare the model fit according to two mathematical formulations with contrasting underlying theoretical assumptions. The first formulation assumes that stomata closure follows a reduction in the capacity to fixate carbon, whereas the second assumes that it follows reduced internal conductance. We compare the fit of these formulations to our data and found that the first formulation (the one assuming that reduced carboxylation capacity underlies the carbon cost of stomatal opening) provided a better fit for all plant functional types, regardless of water stress and leaf ontogeny.
Finally, the parameter of the formulation that provides the best fit for the coupling of stomata opening and photosynthesis serves as proxy for plant water use efficiency and can be readily incorporated onto global dynamic vegetation models to predict vegetation carbon and water exchange under future climatic scenarios for different plant functional types.
We present these results in detail in our paper recently published in open access in Journal of experimental botany. You can read the whole story here.