2019 ASHS Annual Conference

Plants redistribute water between root sections and soil layers along a gradient of decreasing water availability. Therefore, water can be transported from roots in wet soil to roots in dry soil, delaying the onset of water stress and increasing root longevity in dry environments. Grapevines are thought to redistribute water laterally across the trunk from wet to dry portions of the root system. However, it is unknown whether the phloem contributes to such water redistribution. The hypothesis of this study was that hydraulic redistribution in grapevines is in part due to water movement from wet roots to the leaves via the xylem and recycling from the leaves to dry roots via the phloem This study used deuterium-labeled water (²H₂O) as a tracer of water movement. Own-rooted Vitis vinifera L. cv. Merlot grapevines were grown in three-way split root pots. One of the three compartments was irrigated with ²H₂O and the other two were left to dry. The trunk in one of the dry compartments was girdled and the other one was left intact to distinguish between xylem and phloem water movement. Xylem sap and phloem sap, trunk and root tissue, and soil samples were collected. Water was extracted via a cryogenic method and analyzed for deuterium enrichment (δ²H). Following ²H₂O supply to the roots, strong ²H enrichment was found in both xylem and phloem sap collected from petioles. Moreover, the δ²H values were significantly higher in root and soil samples collected from the dry/intact compartment than in samples from the dry/girdled compartment. These results indicate water moves from roots in wet soil to leaves via the xylem and recycles from leaves to roots in dry soil via the phloem. This xylem-to-phloem redistribution in drought-stressed grapevines keeps roots in dry soil alive, as long as a portion of the root system has access to soil water.