Towards a unified theory of plant photosynthesis and hydraulics

The global carbon and water cycles are governed by the coupling of CO2 uptake and water vapour loss through plant leaves, mediated by adaptations that balance carbon gain against hydraulic risk. The authors introduce a trait-based optimality theory that unifies stomatal responses with biochemical acclimation to environments changing across multiple timescales. Tested against experimental drought data from 18 species, the model predicts the simultaneous decline in assimilation rate, stomatal conductance and photosynthetic capacity during progressive soil drought, as well as dependencies on vapour pressure deficit, temperature and CO2. Predictions align with empirical patterns of hydraulic strategies. This unified theory supports reliable modelling of interactive effects of drying soils and rising atmospheric CO2 on global photosynthesis and transpiration.