Hydraulic redistribution: limitations for plants in saline soils.

Nadia Bazihizina,Erik J Veneklaas,Timothy D Colmer,Edward G Barrett‐Lennard

doi:10.1111/pce.13020

Abstract

Hydraulic redistribution (HR), the movement of water from wet to dry patches in the soil via roots, occurs in different ecosystems and plant species. By extension of the principle that HR is driven by gradients in soil water potential, HR has been proposed to occur for plants in saline soils. Despite the inherent spatial patchiness and salinity gradients in these soils, the lack of direct evidence of HR in response to osmotic gradients prompted us to ask the question: are there physical or physiological constraints to HR for plants in saline environments? We propose that build-up of ions in the root xylem sap and in the leaf apoplast, with the latter resulting in a large predawn disequilibrium of water potential in shoots compared with roots and soil, would both impede HR. We present a conceptual model that illustrates how processes in root systems in heterogeneous salinity with water potential gradients, even if equal to those in non-saline soils, will experience a dampened magnitude of water potential gradients in the soil-plant continuum, minimizing or preventing HR. Finally, we provide an outlook for understanding the relevance of HR for plants in saline environments by addressing key research questions on plant salinity tolerance.

Highlights

When different parts of the root system of a single plant are subject to contrasting soil water contents, there can be a transfer of water from moist to dry soil via the roots (Richards & Caldwell 1987)
By contrast with these situations in which water potential gradients are dominated by differences in soil matric potential, we propose that Hydraulic redistribution (HR) is very limited in plants in saline environments where differences in soil salinity result in external osmotic gradients
In this Opinion Paper, we addressed two key questions: (i) are there additional plant internal constraints to HR in saline soils; and (ii) are the osmotic gradients in the plantsoil continuum in saline environments of sufficient magnitude to drive HR? our ability to definitively answer these questions is limited by gaps in knowledge as identified in this Opinion Paper, we advance the hypothesis that HR is unlikely to provide an immediate benefit to plant productivity in spatially heterogeneous saline soils, where osmotic gradients are an important component of the soil water potential gradient within the root zone

Summary

Introduction

When different parts of the root system of a single plant are subject to contrasting soil water contents, there can be a transfer of water from moist to dry soil via the roots (Richards & Caldwell 1987). Plants in spatially heterogeneous saline soils with osmotic-dominated gradients in water potential appear to have little HR.

Results

Conclusion