Optimal stomatal control in the presence of leaf-atmosphere coupling

Plant leaves absorb solar radiation and carbon dioxide (CO₂) from the atmosphere while releasing water vapour, oxygen and heat to the atmosphere. The leaf-atmosphere interface is hence the primary determinant of water-carbon interactions, where stomata control transpiration according to soil water availability, but at the cost of reducing carbon uptake by photosynthesis. It has been proposed that stomata not only respond to water stress, but function in a way to maximise a plant's long-term carbon gain by dynamically economising plant available water according to varying environmental conditions (Cowan and Farquhar, 1977). While the search for the relevant costs of stomatal opening focuses more and more on the costs of the infrastructure needed to supply water to the leaves, the consequences of opening stomata in the presence of leaf-atmosphere feedbacks, potentially resulting in a cooling and humidification of the air at the diurnal scale, hence reducing evaporative demand (Cowan, 1978), and/or depletion of atmospheric CO₂, hence reducing CO₂ uptake, have so far not been considered in stomatal optimality modelling. It has been shown that optimal response of vegetation to even small long-term variations in atmospheric CO₂ can lead to substantial changes in land-atmosphere exchange (Schymanski et al., 2015), while the effect of trends in atmospheric vapour pressure concentration and temperature has also been documented widely. However, little research has been conducted on the optimal behaviour of plants in the presence of land-atmosphere feedbacks.

Here we present a theoretical analysis and preliminary experimental results of (optimal) stomatal control in the presence of leaf-atmosphere coupling. The coupling strength is represented theoretically by adding an additional control volume representing the leaf boundary layer or canopy air space, and experimentally by varying the flow rate of dry and CO₂-rich air into a leaf cuvette. We discuss the positive and negative effects of a de-coupled canopy air space for leaf gas and energy exchange, and present experimental and mathematical methods to put them into relation to each other.

Literature:

Cowan, I. R.: Water use in higher plants, in: Water: planets, plants and people, edited by: McIntyre, A. K., Australian Academy of Science, Canberra, 71–107, 1978.

Cowan, I. R. and Farquhar, G. D.: Stomatal Function in Relation to Leaf Metabolism and Environment, in: Integration of activity in the higher plant, edited by: Jennings, D. H., Cambridge University Press, Cambridge, 471–505, 1977.

Schymanski, S. J., Roderick, M. L., and Sivapalan, M.: Using an optimality model to understand medium and long-term responses of vegetation water use to elevated atmospheric CO2 concentrations, AoB Plants, 7, plv060, https://doi.org/10.1093/aobpla/plv060, 2015.