Coordinated decline of leaf hydraulic and stomatal conductances under drought is not linked to leaf xylem embolism for different grapevine cultivars

Drought decreases water transport capacity of leaves and limits gas exchange, which involves reduced leaf leaf-hydraulic conductance (K-leaf) in both the xylem and outside-xylem pathways. Some literature suggests that grapevines are hyper-susceptible to drought-induced xylem embolism. We combined Kleaf and gas exchange measurements, micro-computed tomography of intact leaves, and spatially explicit modeling of the outside-xylem pathways to evaluate the role of vein embolism and K-leaf in the responses of two different grapevine cultivars to drought. Cabernet Sauvignon and Chardonnay exhibited similar vulnerabilities of K-leaf and g(s) to dehydration, decreasing substantially prior to leaf xylem embolism. K-leaf and g(s) decreased by 80% for both cultivars by Psi(leaf) approximately -0.7 MPa and -1.2 MPa, respectively, while leaf xylem embolism initiated around Psi(leaf) = -1.25 MPa in the midribs and little to no embolism was detected in minor veins even under severe dehydration for both cultivars. Modeling results indicated that reduced membrane permeability associated with a Casparian-like band in the leaf vein bundle sheath would-explain declines in K-leaf of both cultivars. We conclude that during moderate water stress, changes in the outside-xylem pathways, rather than xylem embolism, are responsible for reduced K-leaf and g(s). Understanding this mechanism could help to ensure adequate carbon capture and crop performance under drought.