Role of xylem in root hydraulics: Functionality and implications for drought adaptation

Abstract

Root water transport has been viewed as primarily limited by the radial component, with the axial pathway considered highly conductive and non-limiting. This is supported by theoretical estimates of axial conductance using the Hagen-Poiseuille equation. However, increasing evidence indicates that actual axial conductance is often nearly an order of magnitude lower than predicted, challenging assumptions that it does not limit water uptake. In this review, we discuss how recent model inversion approaches, guided by root hydraulic conductance measurements, have revealed that water transport can be co-limited by radial and axial conductance. We explore possible explanations for this co-limitation, with particular attention to root topology. Finally, we highlight how drought-induced adjustments in xylem vessel traits can reduce axial conductance, contributing to water conservation and cavitation resistance, thereby supporting drought adaptation strategies. Leveraging this overlooked limitation opens new avenues for breeding crops with improved water-use efficiency and resilience to drought .

Keywords: Hagen–Poiseuille; axial conductance; drought; root topology; xylem.

Graphical abstract

Figure 1.

Overview of architectural, anatomical and functional differences in terms of water flow between dicot and monocot-like root systems. The anatomical cross-sections represent different maturation stages along the primary root axis.

Figure 2.

The effect of axial conductance (k _x) and radial conductivity (k _r) changes on whole root system conductance (K _rs) for dicot (soybean, left panels) and monocot (wheat, right panels) species. (a) Effect of k _x and k _r changes (increase or decrease) on modelled K _rs at different root system ages. Based on a default parametrization (Doussan et al., , Baca Cabrera et al., 2024), k _x or k _r were lowered or increased by one order of magnitude for all root types. (b) Heat map with the effect of k _r/k _x ratio changes on K _rs at different root system ages (yellow indicates higher, magenta lower K _rs values). (c) Contrasting root system architecture at the end of the simulations.

Figure 3.

Impact of drought stress on axial conductance (k _x) and potential implications for plant water use and resistance to cavitation. (a) A common response to drought observed in various species is a reduction in metaxylem diameter, which subsequently decreases axial hydraulic conductance (k _x). The figure was created with Biorender.com. (b) This reduction in k _x may support water-saving strategies during the vegetative phase, enabling more conservative water use. As a result, more water may remain available during the reproductive stage, which is critical for reproduction and grain filling. (c) A smaller xylem diameter may also reduce the risk of cavitation. This is because the xylem water potential threshold at which 50% of conductivity is lost due to cavitation tends to become more negative, indicating improved resistance to embolism under water stress.