Hydraulic redistribution and water movement mechanisms in grapevines

Abstract

Context and purpose of the study – Plants have been shown to redistribute water between root sections and soil layers along a gradient of decreasing water availability. One benefit of this hydraulic redistribution is that water can be transported from roots in wet soil to others 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 objectives of the present study were: (1) to determine the pathways of water transport through the vine form wet soil areas to the dry areas; (2) to determine the potential phloem contribution to this water movement.

Material and methods – 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 xylem and phloem water movement. Xylem sap and phloem sap, trunk and root tissue, and soil samples were collected. Water from each sample was extracted via a cryogenic method and analyzed for deuterium enrichment (δ2H).

Results – Following ²H₂O supply to the roots, strong deuterium enrichment was found in both xylem and phloem sap collected from petioles. Moreover, the δ2H values were significantly higher in root tissues and soil 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. The success of irrigation strategies such as partial rootzone drying may be linked to this physiological process.