Rethinking economic theories of plant water use

A central assumption in plant ecophysiology is that carbon is the primary currency for plant fitness. To this end, plants are thought to maximize carbon gain and any deviations from maximum carbon gain are ascribed to resource limitations (e.g., temperature, drought), biophysical limitations (e.g., biophysical limits on cell size), or variation in plant life history that may prioritize future carbon gain over current carbon gain (i.e., applying an economic discount rate to carbon). Compared to living in water, living on land made accessing CO₂ substantially easier: CO₂ diffuses approximately 10,000 times faster in air than in water. However, because this CO₂ must diffuse into the aqueous environment of the living mesophyll cells where photosynthetic metabolism occurs (The´roux-Rancourt et al. 2021), the greater CO₂ supply of the terrestrial lifestyle also comes with a cost: losing approximately 200-400 molecules of water by transpiration for every molecule of CO₂ fixed by photosynthesis (Nobel et al. 2005). Water, therefore, is considered a valuable resource to be conserved and not wasted. As such, much of the field of plant ecophysiology posits carbon as the central currency for which water is traded.