Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees

Authors: GARDNER, ANNA - University Of Birmingham; JIANG, MINGKAI - Western Sydney University; ELLSWORTH, DAVID - Western Sydney University; MACKENZIE, A ROBERT - University Of Birmingham; PRITCHARD, JEREMY - University Of Birmingham; BADER, MARTIN - Western Sydney University; BARTON, CRAIG - Auckland University Of Technology; Bernacchi, Carl; CALFAPIETRA, CARLO - Consiglio Nazionale Delle Ricerche; CROUS, KRISTINE - Western Sydney University

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2022
Publication Date: 11/13/2022
Citation: Gardner, A., Jiang, M., Ellsworth, D., Mackenzie, A., Pritchard, J., Bader, M., Barton, C., Bernacchi, C.J., Calfapietra, C., Crous, K., et al. 2022. Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees. New Phytologist. 237(4):1229-1241. https://doi.org/10.1111/nph.18618.
DOI: https://doi.org/10.1111/nph.18618

Interpretive Summary: Leaves of plants contain small openings called stomata from which gases, including carbon dioxide and water vapor, are exchanged with the atmosphere. Plants are continually balancing carbon uptake by leaves for growth via photosynthesis and water loss from leaves via transpiration. This research tests the theory that plants, in this case trees specifically, optimize the amount of water lost relative to carbon gained and whether this strategy applies to two global change factors - both rising atmospheric carbon dioxide and drier atmospheric conditions. This study combines a large number of previously published datasets into what is termed a metaanalysis. The results show that increases in carbon dioxide drove higher photosynthetic rates (more carbon going into the plant) but that the stomata partially closed meaning less water was lost from the leaves. Drier atmospheric conditions also increased photosynthesis responses to higher carbon dioxide. Thus, the amount of water that is lost through the leaves is optimized when plants are grown in higher carbon dioxide particularly with drier atmospheric conditions. These results were generally consistent for the different tree species studied and the results indicate the potential for a unified stomatal model to be incorporated into models of tree and ecosystem function with global change.

Technical Abstract: • Optimal stomatal theory predicts stomata are regulated to maximize photosynthesis and minimize transpiration to achieve optimal intrinsic water-use efficiency (iWUE). However, it remains uncertain how elevated atmospheric CO2 (eCO2) and leaf-air vapour pressure deficit (D) affect iWUE, and the relative contribution of net photosynthesis (Anet) and stomatal conductance (gs). • We conducted a meta-analysis of tree studies to examine the effect of eCO2 on iWUE, gs and Anet in light-saturated conditions. We compared three plant functional types (PFTs) and utilised the Unified Stomatal Optimisation (USO) model to explore observations. We hypothesised that D would affect the sensitivity of plant iWUE to eCO2 and therefore the USO model can be used to predict the response of iWUE to eCO2. We expect a smaller gs response to eCO2, alongside a greater Anet response, in gymnosperms when compared to angiosperms. • For most species, iWUE increased in proportion to increases in eCO2 and the effect of increased Anet was larger than the effect of reduced gs. D significantly increased Anet and iWUE responses to eCO2 in angiosperms, compared to gymnosperms. The USO model correctly captured stomatal behaviour with eCO2 across most PFTs. • Land surface models can utilize the USO model to depict stomatal behaviour under changing atmospheric conditions for most of the PFTs examined. However, further assessment of the model is required for boreal and tropical species.