Experimental and conceptual approaches to root water transport

Abstract

Background: Root water transport, which critically contributes to the plant water status and thereby plant productivity, has been the object of extensive experimental and theoretical studies. However, root systems represent an intricate assembly of cells in complex architectures, including many tissues at distinct developmental stages. Our comprehension of where and how molecular actors integrate their function in order to provide the root with its hydraulic properties is therefore still limited.

Scope: Based on current literature and prospective discussions, this review addresses how root water transport can be experimentally measured, what is known about the underlying molecular actors, and how elementary water transport processes are scaled up in numerical/mathematical models.

Conclusions: The theoretical framework and experimental procedures on root water transport that are in use today have been established a few decades ago. However, recent years have seen the appearance of new techniques and models with enhanced resolution, down to a portion of root or to the tissue level. These advances pave the way for a better comprehension of the dynamics of water uptake by roots in the soil.

Keywords: Biophysics; Forward genetics; Modeling; Pressure chamber; Root pressure probe; Root water transport.

Fig. 1

Schematic representation of experimental setups dedicated to measuring whole root water transport properties. A: pressure chamber, B: High Pressure Flow Meter (soil texture designed by brgfx / Freepik), C: exudation, D: root pressure probe ( adapted from Bramley et al. 2007). A-C: The motive force is presented bold purple, the measured flow in purple. Flow can be measured in various ways such as a graduated capillary, a balance, or a flowmeter

Fig. 2

Illustrations of measurements of hydraulic properties at the cell level. A: drawing of a cell pressure probe, B: example pictures of protoplast before (left pictures) and after a hypo-osmotic shock (pipette diameter ~ 10 µm), C: drawing of a stopped-flow spectrophotometer

Fig. 3

Representative root water transport models in relation to their structure or scale. The first column essentially refers to models that use a two compartments analogy (soil and xylem), representing either the whole root system or radial water transport. The second column presents models as a network of resistances that allow to recapitulate radial water transport. The third column (“root segment”) cites models that address the hydraulic function of a root segment or of an unbranched root (it may share similarities with the first column). Finally, the last column presents models that aim at calculating the water flow over an entire and highly branched root system. Note that this figure presents a non-exhaustive list of models