Leaves lose approximately 400 H2O molecules for every 1 CO2 gained during photosynthesis. Most long-distance water transport in plants, or xylem sap flow, serves to replace this water to prevent desiccation. Theory predicts that the largest vessels contribute disproportionately to overall sap flow because flow in pipe-like systems scales with the fourth power of radius. Here, we confront these theoretical flow predictions for a vessel network reconstructed from X-ray μCT imagery with in vivo flow MRI observations from the same sample of a first-year grapevine stem. Theoretical flow rate predictions based on vessel diameters are not supported. The heterogeneity of the vessel network gives rise to transverse pressure gradients that redirect flow from wide to narrow vessels, reducing the contribution of wide vessels to sap flow by 15% of the total. Our results call for an update of the current working model of the xylem to account for its heterogeneity.

Department of Viticulture and Enology University of California 595 Hilgard Ln Davis CA 95616 USA

IBG 2 Plant Sciences Forschungszentrum Jülich Leo Brandt Straße 1 52428 Jülich Germany

Institute of Botany of the Czech Academy of Sciences Zámek 1 25243 Průhonice Czech Republic

School of Forestry and Environmental Studies Yale University 195 Prospect St New Haven CT 06511 USA

USDA ARS Crops Pathology and Genetics Research Unit Davis CA USA

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