Experimental Assessment of Optimal Stomatal Control under Drought and Heat Stress

Stomata, small openings on leaves, are critical for regulating the exchange of water vapor (transpiration) and carbon dioxide (assimilation) with the atmosphere. Thus, stomata serve as a key interface between plant physiological processes and ecosystem water and carbon fluxes. The "optimal stomatal control" hypothesis suggests that stomatal behavior is optimized dynamically in a way to achieve maximum carbon uptake given limited water availability for transpiration between rainfall events. The theoretically optimal stomatal conductance follows a consistent slope λ between transpiration (E) and carbon assimilation (A), i.e. λ = ∂E/∂A.

Due to the inability to measure λ directly, the theory has only been tested by fitting leaf gas exchange measurements to models of photosynthesis and transpiration, with the frequent outcome of apparently strongly varying and inconsistent values of λ. However, it is unclear whether such results are due to model and measurement uncertainty or indeed contradict the theory of optimal stomatal control.

After developing an experimental approach to measure λ directly at the leaf scale, we investigate how far λ is consistent between leaves of the same plant or even between plants, under unstressed, single-stress, and combined stress of heat and drought conditions. By combining our measurements with classical leaf gas exchange modeling, we bridge the gap between experimental and theoretical studies of stomatal optimization. Additionally, we measure water use efficiency, photosynthetic capacity, and biomass to link stomatal control mechanisms to plant physiological functioning. Our results provide insights into how stomatal response to heat and drought stress influences water-carbon trade-offs at the leaf level. Scaling up leaf-level behavior to the ecosystem, e.g. with the help of terrestrial biosphere models, opens new possibilities for predicting water resource dynamics, optimizing water resource management in agricultural systems, and improving ecosystem management strategies. Assessment of stomatal control strategies of different plants can also enhance our ability to select heat- and drought-resilient crop varieties for a changing environment.