Publication

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.