Table 1. Modeling framework across scales to determine the underlying mechanisms linking Kleaf decline to gas exchange.
Aarea, Leaf photosynthetic rate; gmax, maximum stomatal conductance; gmin, minimum epidermal conductance; Kx, leaf xylem hydraulic conductance; PLC, percentage loss of hydraulic conductance.
| Model | Purpose | Input | Output | Results |
|---|---|---|---|---|
| K_LEAF (Cochard et al., 2004; Scoffoni et al., 2017a) | Model the influence of xylem embolism and potential conduit collapse on Kx and Kleaf | Leaf size, number of secondary veins, and theoretical conductivities from the different vein orders (1) at full turgor and after accounting (2) for the decline caused by the observed embolism in midrib and/or secondary veins and (3) for the decline potentially caused by collapsed xylem conduits of tertiary and higher order veins (under a realistic collapsed scenario as observed in red oak species [Zhang et al., 2016], which caused 13% PLC, and a more severe scenario causing 50% PLC) | Kx | Neither embolism nor xylem conduit collapse caused a decline in Kx substantial enough to explain the observed decline in Kleaf |
| MOFLO 2.0 (Buckley et al., 2017) | Model the influence of changes in outside-xylem pathways on Kox and Kleaf | Cell shrinkage and percentage intercellular airspace at −0.5 MPa obtained from microCT, gs (abaxial and adaxial), VPD, and bulk leaf temperature; simulations were performed under no light or 600 μmol m−2 s−1 photosynthetically active radiation, with or without an apoplastic barrier at the bundle sheath and with or without an 80% decline in cell membrane permeability and/or cell connectivity | Kox | Reduction of cell membrane permeability in the context of an apoplastic barrier would account for most of the Kleaf decline observed at −0.5 MPa; temperature gradients through the leaf due to irradiance had little impact on Kox |
| Marginal contribution of K decline (refined from Rodriguez-Dominguez et al., 2016) | Quantify the influence of Kleaf decline on gs decline | Parameters from the maximum likelihood function of gs and Kleaf versus Ψleaf, VPD set at a constant value (1.5 kPa), and a computed range of percentage gs decline (0%–100% decline in gs with Ψleaf) | Contribution of Kleaf decline to gs decline with dehydration | Kleaf decline explains most of the changes in gs during mild to moderate dehydration |
| SurEau (Martin-StPaul et al., 2017) | Quantify the influence of Kleaf decline on gas exchange in the whole-plant context during drought | Parameters from the maximum likelihood function of Kleaf versus Ψleaf, parameters from the function of Kroot versus water potential, gmin, gmax, Farquhar’s model inputs, photosynthetically active radiation, air temperature, air humidity, time of day, transpiration under well-hydrated conditions, and soil volume | Soil water reserve, water potentials, transpiration rate, gs, Aarea, and PLC | Decline in Kleaf causes Ψleaf to drop, which in turn causes both gs and Aarea to decline under increasing VPD and decreasing soil water potential |