The humidity inside leaves and why you should care: implications of unsaturation of leaf intercellular airspaces

We are unaware of published measurements of smes in P. edulis, but smes varied from 14.4 to 40 m2 m-2 across 15 diverse species in a study by Tomas et al. (2013). We adopt smes = 10 m2 m-2 to ensure the resulting estimate of Pm is not an underestimate due to uncertainty in this parameter. Applying Eq. S6 to the data of Cernusak et al. (2018) indicates that, to sustain a symplastic water potential of -2 MPa while water flows across the cell membranes to an apoplastic space with water potential given by Eq. S1 would require Pm as low as 1.710-9 m s-1, or 0.0017 m s-1, for P. edulis, and as low as 9.810-8 m s-1, or 0.0098 m s-1, for J. monosperma (Appendix S3a). For P. edulis, 25% of measurements indicate Pm at least as small as 0.0083 m s-1; for J. monosperma, 25% of measurements indicated Pm < 0.099 m s. By comparison, mean values of Pm previously reported for mesophyll protoplasts are orders of magnitude larger (e.g., 5–25 m s-1; Shatil-Cohen et al., 2013).