Transport in Plants

Lessons

Senior 5 · Biology

Transport in Plants

4 lessons

Lesson 2 of 4

Water uptake by roots and its movement through the plant

25 min

A wilting bean plant stands up again overnight

On a hot afternoon in Bugesera, a row of bean plants droops. A farmer waters the soil at dusk, and by morning the leaves stand firm again, although nobody poured water on the leaves. The water must have entered the roots, crossed them, and climbed the stem against gravity. In Unit 2 you met water potential and osmosis in single cells. This lesson uses the same ideas to follow water through a whole plant, and asks what force can lift it to the top of a tree with no pump at all.

Definition

Water potential ( $\psi$ , "psi"): a measure of the tendency of water to move out of a solution, in kilopascals (kPa) or megapascals (MPa). Pure water at atmospheric pressure has $\psi = 0$ ; dissolving solutes makes it negative. Water always moves from a higher (less negative) to a lower (more negative) water potential.

Apoplast pathway: the route through the cell walls and the spaces between cells, without crossing any membrane.

Symplast pathway: the route through the cytoplasm of cells, passing from cell to cell through plasmodesmata.

Casparian strip: a band of waterproof suberin in the radial and end walls of endodermis cells that blocks the apoplast pathway.

Root pressure: a positive pressure in the root xylem caused by the active pumping of ions into it, which draws water in by osmosis.

Cohesion: the attraction between water molecules, by hydrogen bonds. Adhesion: the attraction between water molecules and another surface, such as the wall of a xylem vessel.

Step 1: into the root hair

Each root hair is a long, thin outgrowth of one epidermal cell, a few millimetres behind the root tip. Thousands of them give the root a huge surface area in close contact with the soil water. Their walls are thin and they have no waxy cuticle.

Water enters by osmosis. The cell sap contains sugars and ions, so its water potential is lower than that of the dilute soil water.
Mineral ions, such as nitrate and potassium, are usually more concentrated inside the root hair than in the soil, so they are taken up by active transport through carrier proteins, using ATP from respiration. Evidence: uptake of ions stops if the roots are deprived of oxygen or treated with a respiratory poison such as cyanide, but the uptake of water by osmosis continues for a while.

Worked example 1: why too much fertiliser makes a crop wilt

The water potential of a root hair cell is $-400\,\mathrm{kPa}$ . (a) The soil water has $\psi = -150\,\mathrm{kPa}$ . Which way does water move? (b) A farmer spreads far too much fertiliser by one row of cabbages, and the soil water becomes $-650\,\mathrm{kPa}$ . What happens now, and why?

(a) Water moves from higher to lower water potential. $-150\,\mathrm{kPa}$ is higher than $-400\,\mathrm{kPa}$ , so water moves into the root hair by osmosis. The gradient is $-150 - (-400) = 250\,\mathrm{kPa}$ .
(b) Now the soil, at $-650\,\mathrm{kPa}$ , is lower than the root hair, at $-400\,\mathrm{kPa}$ . Water moves out of the root into the soil.
The cells lose turgor and the plants wilt even though the soil is wet. This is "fertiliser burn". The cure is to water heavily so as to dilute the soil solution, and to follow the recommended dose.

Step 2: across the root to the xylem

From the root hair, water crosses the cortex towards the centre of the root by two main routes (Figure 4):

The apoplast pathway: through the cellulose cell walls, which are full of water-filled spaces. Water is pulled along by cohesion as water ahead of it moves into the xylem. It meets little resistance, so in the cortex most water moves this way.
The symplast pathway: into the cytoplasm of the root hair, then from cell to cell through the plasmodesmata, down a water potential gradient. (Some books add a vacuolar pathway, from vacuole to vacuole across the membranes, which is slower.)

The endodermis is the checkpoint. The Casparian strip in the endodermis walls is waterproof, so the apoplast pathway stops there. Water and ions must cross a cell surface membrane and enter the cytoplasm of an endodermis cell. This lets the plant choose which ions enter its xylem, and keep out toxic ions and soil pathogens.

Endodermis and pericycle cells actively pump mineral ions into the xylem. This lowers the water potential of the xylem sap, so water follows by osmosis. Because water cannot leak back out through the apoplast, a pressure builds up: root pressure. You can see it when a stem cut near the ground oozes sap, and in guttation, when drops of water appear at the edges of grass leaves on a still, humid morning. Root pressure stops if the roots are cooled or poisoned with cyanide, which shows that it depends on active transport.

Step 3: up the stem, by cohesion and tension

The main force that lifts water is not in the root at all, but in the leaves. The cohesion–tension theory explains it in four steps.

Evaporation. Water evaporates from the wet walls of the spongy mesophyll cells into the air spaces of the leaf, and diffuses out through the stomata. This is transpiration (lesson 3).
Water leaves the xylem. The mesophyll cells' water potential falls, so water moves into them from the leaf xylem, through the apoplast and the symplast.
Tension. Losing water from the top of the xylem pulls on the water column below, putting it under tension (a negative pressure).
Cohesion and adhesion. Hydrogen bonds hold the water molecules together, so the whole column is pulled up as one unbroken thread, from root to leaf. Adhesion to the lignified walls also helps to stop the column pulling away from the sides.

The whole pathway is a gradient of water potential, from the soil to the air (Figure 5). The steepest step by far is from the leaf into the air, which is why the sun's energy, evaporating water from the leaves, drives the flow. The plant spends no ATP on lifting water.

The evidence

The diameter of a tree trunk, measured with a fine gauge, shrinks slightly at midday, when transpiration is fastest: the xylem walls are pulled inwards by the tension.
If a xylem vessel in a transpiring shoot is cut, air is drawn in rather than water leaking out. A cut flower stem placed in coloured water draws the dye up into its petals.
In the morning, the flow of sap speeds up in the twigs first and in the base of the trunk later, as it would if it is pulled from the top.
Any break in the column, such as an air bubble (an embolism), stops flow in that vessel. Pits let water bypass it into neighbouring vessels.

Other forces are minor. Capillarity, the rise of water in narrow tubes by adhesion, lifts water less than a metre in xylem vessels. Root pressures are usually well below 1 MPa and appear mainly at night or when the soil is wet and transpiration slow; while trees transpire fast on a sunny day, their xylem sap is under tension, not pressure.

Worked example 2: could root pressure push water up a eucalyptus?

A eucalyptus tree in a woodlot is 30 m tall. (a) What pressure is needed just to hold up a column of water 30 m high? Use $p = \rho g h$ , with $\rho = 1\,000\,\mathrm{kg\,m^{-3}}$ and $g = 9.81\,\mathrm{N\,kg^{-1}}$ . (b) A root pressure of 0.10 MPa is measured in a seedling. Could a pressure this size lift water to the top?

(a) $p = \rho g h = 1\,000 \times 9.81 \times 30 = 2.94 \times 10^{5}\,\mathrm{Pa} \approx 0.29\,\mathrm{MPa}$
(b) The height a pressure of 0.10 MPa can support is $h = \frac{p}{\rho g} = \frac{1.0 \times 10^{5}}{1\,000 \times 9.81} = 10\,\mathrm{m}$
That is only a third of the tree's height, even before the friction of the moving sap is counted.

Conclusion: root pressure of this size cannot be the main force. The leaves must pull the water up, with tensions in the leaf xylem of the order of $- 1$ to $-2\,\mathrm{MPa}$ on a sunny day.

Warning

Common mistakes

"Water enters root hairs by active transport." Water enters by osmosis; it is the mineral ions that are actively transported.
Mixing up cohesion and adhesion. Cohesion is water sticking to water; adhesion is water sticking to the vessel wall.
"The Casparian strip stops water entering the xylem." It stops water moving through the cell walls of the endodermis; water still crosses through the endodermis cells.
"Water moves to a higher water potential." It always moves to a lower (more negative) one. −600 kPa is lower than −200 kPa.

Cognito: "A-Level Biology - Water Transport in Plants - Root Pathways, Xylem, Transpiration". Watch for the apoplast and symplast routes and the role of the Casparian strip, then for how cohesion and tension pull the water column up the xylem; compare them with Figures 4 and 5.

Tip

Memory hook: Apoplast = Along the walls; Symplast = through the Stuff inside (cytoplasm). And for the theory, think of sucking a drink through a straw: the pull comes from the top (evaporation), and the drink rises as one column because it holds together (cohesion).

✦

Practice: water uptake and movement

State two features of a root hair cell that help it to absorb water.
Distinguish between the apoplast and the symplast pathways.
Explain why the uptake of potassium ions by roots stops when the soil becomes waterlogged and short of oxygen.
A root hair cell has $\psi = -350\,\mathrm{kPa}$ and the next cortex cell $-480\,\mathrm{kPa}$ . In which direction does water move between them, and down what gradient?
Explain the role of the Casparian strip.
Calculate the pressure needed to support a water column 45 m tall ( $\rho = 1\,000\,\mathrm{kg\,m^{-3}}$ , $g = 9.81\,\mathrm{N\,kg^{-1}}$ ). Give your answer in MPa to 2 significant figures.
A student says, "Root pressure lifts water to the leaves." Use two pieces of evidence to argue against this.
Suggest why a long air bubble in a xylem vessel does not stop the whole plant from receiving water.

Show answersHide answers

Any two: a long thin extension gives a large surface area; a thin wall and no cuticle give a short diffusion path; cell sap with a low water potential.
Apoplast: through the cell walls and spaces between cells, crossing no membranes. Symplast: through the cytoplasm, from cell to cell by plasmodesmata.
Potassium is taken up by active transport, which needs ATP from aerobic respiration; without oxygen the roots make too little ATP.
From the root hair (−350 kPa) to the cortex cell (−480 kPa), down a water potential gradient of 130 kPa.
It is a waterproof band of suberin in the endodermis walls that blocks the apoplast pathway, so water and ions must pass through the endodermis cytoplasm. This lets the plant control which ions reach the xylem, and lets root pressure build up.
$p = 1\,000 \times 9.81 \times 45 = 4.41 \times 10^{5}\,\mathrm{Pa} \approx 0.44\,\mathrm{MPa}$ .
Any two: typical root pressures support only about 10 m of water, less than the height of tall trees; in transpiring trees the xylem sap is under tension, so a cut vessel draws in air rather than leaking; trunk diameter shrinks at midday; flow starts in the twigs before the trunk base.
Water can move sideways through the pits into neighbouring vessels and go around the blocked one; there are thousands of parallel vessels.

Key points

Water enters root hairs by osmosis; mineral ions enter by active transport.
Water crosses the cortex mainly by the apoplast pathway and also by the symplast; the Casparian strip forces it through the endodermis cytoplasm.
Active pumping of ions into the xylem causes root pressure, which is small.
The cohesion–tension theory: evaporation from the leaves creates tension that pulls a continuous, cohesive column of water up the xylem; the energy comes from the sun, not from ATP.

Sources and credits

Written by Ganzaa. Facts were checked against 30.5 Transport of Water and Solutes in Plants, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax, licensed under CC BY 4.0. No text was copied.
Photo: Arabidopsis root hairs, G Lens Microscopist, Megan Gerber, CC BY 4.0, via Wikimedia Commons.
Video: A-Level Biology - Water Transport in Plants - Root Pathways, Xylem, Transpiration, Cognito, embedded from YouTube.

Figures 4 and 5, the worked examples and the practice questions are by Ganzaa. The water potentials in Figure 5 and Worked example 1 are illustrative.

1A wilting bean plant stands up again overnight

2Step 1: into the root hair

3Step 2: across the root to the xylem

4Step 3: up the stem, by cohesion and tension

The evidence

A wilting bean plant stands up again overnight

Step 1: into the root hair

Step 2: across the root to the xylem

Step 3: up the stem, by cohesion and tension