Species’ differences in the stringency of stomatal control of plant water potential represent a continuum of iso- to anisohydric behavior. However, little is known about how quasi-steady state stomatal regulation of water potential may relate to dynamic behavior of stomata and photosynthetic gas exchange in species operating at different positions along this continuum. We expected that trade-offs associated with variation in biophysical traits and constraints along the continuum would be reflected in both dynamic and quasi-steady state stomatal responses to fluctuating environmental variables. Specifically, we expected faster stomatal kinetics and higher quasi-steady state stomatal conductance with increasing anisohydry. We also expected stomatal and photosynthetic traits to co-vary along this continuum. Thus, if stomatal kinetics are faster and stomatal conductance higher in more anisohydric species, we expected faster activation and higher maximum rates of photosynthesis with increasing anisohydry. We addressed these expectations by characterizing several features of dynamic and quasi-steady state regulation of photosynthetic gas exchange in well-irrigated individuals of ten diverse woody species. These species represent a broad range of iso- to anisohydric behavior based on their leaf water potential at turgor loss, a proxy for degree of anisohydry. We quantified absolute and relative rates of light-induced stomatal opening and activation of photosynthesis, quasi-steady state operating stomatal conductance, CO2 assimilation rates, various measures of photosynthetic capacity and intrinsic water-use efficiency (WUE).
Consistent with our predictions, rates of light-induced stomatal opening and activation of photosynthesis increased several-fold with increasing anisohydry as did quasi-steady state stomatal conductance and maximum photosynthetic rate. Intrinsic WUE decreased with increasing anisohydry indicating reduced relative stomatal limitation of photosynthesis. Kinetics of stomatal opening and activation of photosynthesis were closely coordinated across a continuum of iso- to anisohydry as indicated by a linear relationship between species’ half-times for light-induced stomatal opening and activation of photosynthesis. In comparisons between the preceding functional traits, species rankings were highly consistent, leading to species-independent scaling relationships over the range of anisohydry observed. Conducting the study under relatively uniform conditions of minimal water stress in a greenhouse common garden contributed to our ability to identify genetically-based variation inherent trends in functional traits along a spectrum of iso- to anisohydry. Our results have important implications for understanding multiple functional trade-offs that cause species to operate at different points along a whole-plant economics spectrum of fast to slow strategies in terms of rates of resource acquisition and use.