Abstract
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.
Highlights
Leaves lose approximately 400 H2O molecules for every 1 CO2 gained during photosynthesis
Because the vessels are nearly circular in crosssection, water transport is often approximated with the Hagen–Poiseuille (HP) equation, which describes liquid flow rates in cylindrical pipes where volume flow rate is proportional to the radius raised to the 4th power, such that a small increase in radius leads to a large increase in flow
In vivo flow rates in xylem conduits observed with magnetic resonance velocity flow imaging (MRI) had values of 0.75 ± 0.86 μg s−1 (n = 553 pixels), with a median of only 0.44 μg s−1 and a tail with 4.2% of flows above 3 μg s−1, reaching as high as 5.3 μg s−1 (Fig. 2b, Supplementary Fig. 1)
Summary
Leaves lose approximately 400 H2O molecules for every 1 CO2 gained during photosynthesis. Because the vessels are nearly circular in crosssection, water transport is often approximated with the Hagen–Poiseuille (HP) equation, which describes liquid flow rates in cylindrical pipes where volume flow rate is proportional to the radius raised to the 4th power, such that a small increase in radius leads to a large increase in flow This framework shows good agreement with experimental data on the hydraulic conductivity of excised stem segments if vessels are cut open at both ends and a uniform pressure gradient is applied to the network of vessels[6]. For flow in actual xylem connected to both roots and leaves, maximum theoretical values based on HP predictions for an observed vessel diameter distribution must be reduced by empirical factors accounting for the resistance of the interconduit pit membranes embedded in the perforated end and side walls between adjacent vessels[2]. Successive lumina are connected serially, with no connections across vessel files. b Four vessels (blue) segmented from a 3D stack of μCT images (at base). c Idealization of the same four vessels in the present model: each lumen is represented as circular in cross-section, with radius deduced from the data; connections (red) are placed at the centre of intervals where images show vessels to be connected
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