UC Davis

C₄ plants are major grain (maize [Zea mays] and sorghum [Sorghum bicolor]), sugar (sugarcane [Saccharum officinarum]), and biofuel (Miscanthus spp.) producers and contribute ∼20% to global productivity. Plants lose water through stomatal pores in order to acquire CO₂ (assimilation [A]) and control their carbon-for-water balance by regulating stomatal conductance (g_S). The ability to mechanistically predict g_S and A in response to atmospheric CO₂, water availability, and time is critical for simulating stomatal control of plant-atmospheric carbon and water exchange under current, past, or future environmental conditions. Yet, dynamic mechanistic models for g_S are lacking, especially for C₄ photosynthesis. We developed and coupled a hydromechanical model of stomatal behavior with a biochemical model of C₄ photosynthesis, calibrated using gas-exchange measurements in maize, and extended the coupled model with time-explicit functions to predict dynamic responses. We demonstrated the wider applicability of the model with three additional C₄ grass species in which interspecific differences in stomatal behavior could be accounted for by fitting a single parameter. The model accurately predicted steady-state responses of g_S to light, atmospheric CO₂ and oxygen, soil drying, and evaporative demand as well as dynamic responses to light intensity. Further analyses suggest that the effect of variable leaf hydraulic conductance is negligible. Based on the model, we derived a set of equations suitable for incorporation in land surface models. Our model illuminates the processes underpinning stomatal control in C₄ plants and suggests that the hydraulic benefits associated with fast stomatal responses of C₄ grasses may have supported the evolution of C₄ photosynthesis.