Plant Water Relations and Transport

Plants move enormous quantities of water from soil to atmosphere, and sugars from leaves to growing tissues, using gradients of water potential and pressure rather than a beating pump.

Definition

Plant water relations describe how water moves into, through, and out of plants along gradients of water potential, and transport covers the long-distance movement of water in the xylem and of sugars in the phloem.

Scope

This topic covers water potential and its components, osmosis and turgor, water uptake by roots, the cohesion–tension mechanism of xylem transport, stomatal regulation of transpiration, and the pressure-flow translocation of sugars in the phloem.

Core questions

• How does water potential govern the movement of water between soil, plant, and air?
• What mechanism allows water to be pulled to the top of a tall tree?
• How do stomata balance carbon gain against water loss, and how is sugar translocated in the phloem?

Key theories

Cohesion–tension theory

Transpiration at the leaves places the xylem sap under tension; the strong cohesion of water molecules and their adhesion to vessel walls transmit this tension down to the roots, drawing water upward.

Pressure-flow model of phloem transport

Loading sugars into the phloem at sources lowers water potential and draws water in, raising pressure that drives bulk flow toward sinks, where sugars are unloaded and pressure falls.

Mechanisms

Water moves passively toward lower water potential, the sum of solute (osmotic) and pressure (turgor) potentials. In roots, water crosses to the xylem via apoplastic and symplastic paths regulated by the Casparian strip. Transpiration from mesophyll cells generates negative pressure that the cohesive water column transmits through the xylem to the soil. Stomata, opened by guard-cell turgor, regulate the trade-off between CO2 uptake and water loss. In the phloem, active sugar loading at sources raises turgor and drives pressure-driven bulk flow to sinks.

Clinical relevance

Water relations govern crop drought tolerance and irrigation needs: managing transpiration, stomatal behavior, and water-use efficiency is central to producing food under limited water and to predicting plant responses to a drying climate.

History

Dixon and Joly proposed the cohesion–tension theory of sap ascent around 1894, and Münch's pressure-flow hypothesis in the 1920s explained phloem transport; both remain the accepted frameworks.

Key figures

• Stephen Hales
• Henry Dixon
• Ernst Münch

Seminal works

• taiz2015
• raven2013

Frequently asked questions

What is water potential?

Water potential is a measure of the free energy of water per unit volume; water moves spontaneously from regions of higher to lower water potential, and in plants it combines solute concentration and physical pressure.

Why do stomata close during drought?

Under water stress the hormone abscisic acid triggers loss of guard-cell turgor, closing the stomata to conserve water at the cost of reduced carbon dioxide uptake and photosynthesis.