Transport and water relations in plants: the structure and function of xylem and phloem, water uptake by roots through the apoplast and symplast pathways, the cohesion-tension theory of the transpiration stream, the factors affecting transpiration and how a potometer measures it, translocation by the mass flow hypothesis from source to sink, and the adaptations of xerophytes that reduce water loss.

What this dot point is asking

CCEA wants you to describe the structure and function of xylem and phloem, explain how water is taken up by roots and carried to the leaves by the cohesion-tension theory, explain the factors that affect transpiration and how a potometer is used, explain translocation in the phloem by the mass flow hypothesis from source to sink, and describe the xerophytic adaptations that reduce water loss.

Xylem and phloem structure

Xylem vessels are dead, hollow tubes formed from columns of cells whose end walls have broken down, leaving a continuous channel. Their walls are thickened and waterproofed with lignin, which strengthens them so they do not collapse under the tension of the transpiration stream and stops water leaking out; pits in the walls allow water to move sideways. Because the cells are dead and empty, they offer little resistance to flow.

Phloem is made of living sieve tube elements joined end to end through perforated sieve plates. These cells have very little cytoplasm and few organelles, leaving a clear path for the phloem sap to flow. Each sieve tube element is supported by an adjacent companion cell that has dense cytoplasm and many mitochondria; it supplies ATP and carries out the metabolic work (including active loading) for the sieve tube, the two being linked by plasmodesmata.

Water uptake and the transpiration stream

Water is absorbed from the soil by the root hair cells, which provide a large surface area and a steep water potential gradient (the cell sap has a lower water potential than the soil solution). Water then crosses the root to the xylem by two routes: the apoplast pathway (through the cell walls and intercellular spaces without entering the cytoplasm) and the symplast pathway (from cell to cell through the cytoplasm and plasmodesmata, by osmosis).

The sequence is:

1. Water evaporates from the moist walls of the mesophyll cells and diffuses out through the stomata (transpiration).
2. The water potential of the mesophyll cells falls, so water enters them by osmosis from the xylem, putting the xylem water under tension (negative pressure).
3. Water molecules are attracted to each other by hydrogen bonding (cohesion), so they move as a continuous column, and they adhere to the lignified walls.
4. The tension is transmitted down the whole column, drawing water into the roots from the soil down a water potential gradient. This continuous flow is the transpiration stream.

Factors affecting transpiration and the potometer

Anything that steepens the water vapour gradient or speeds diffusion increases the transpiration rate.

• Light opens the stomata, so the rate increases.
• Temperature raises the kinetic energy of water molecules and the rate of evaporation, so the rate increases.
• Air movement (wind) removes water vapour from around the stomata, keeping the gradient steep, so the rate increases.
• Humidity raises the water vapour concentration outside the leaf, reducing the gradient, so the rate decreases.

A potometer measures the rate of water uptake by a cut leafy shoot, which is taken as a close proxy for transpiration. The apparatus is set up under water to avoid air entering the xylem, the cut is made under water, and an air bubble is introduced into the capillary tube; the distance the bubble moves in a set time gives the rate of uptake.

Translocation by mass flow

Translocation moves assimilates, mainly sucrose, through the phloem from a source to a sink. The mass flow hypothesis explains it as a bulk flow driven by a hydrostatic pressure gradient:

1. At the source, companion cells actively load sucrose into the sieve tubes using ATP. This lowers the water potential there, so water enters from the xylem by osmosis, raising the hydrostatic pressure at the source.
2. At the sink, sucrose is removed (used in respiration or stored), raising the water potential, so water leaves and the hydrostatic pressure falls.
3. The pressure gradient between the high-pressure source and the low-pressure sink drives the mass flow of phloem sap from source to sink.

Xerophyte adaptations

Xerophytes are plants adapted to dry habitats, with features that reduce transpiration: a thick waxy cuticle to cut evaporation; sunken stomata in pits and leaf hairs that trap moist air and reduce the water potential gradient; rolled leaves (as in marram grass) that enclose a humid space around the stomata; and a reduced leaf area (such as spines) that lowers the surface area for water loss. Some store water in fleshy tissues (succulents).

Examples in context

Example 1. Marram grass on sand dunes. Marram grass rolls its leaves so the stomata face inwards into a humid pocket of trapped air, with hairs and sunken stomata reducing the water potential gradient. It is a textbook example of a set of xerophytic adaptations working together to limit water loss on a dry, windy dune.

Example 2. Ringing a tree trunk. Removing a complete ring of bark, which contains the phloem, stops sucrose moving down to the roots. Sugars build up in the bark just above the ring and the roots are eventually starved. This is classic evidence that the phloem, not the xylem, carries assimilates from source to sink.

Try this

Q1. Explain why xylem vessels are lignified. [2 marks]

• Cue. Lignin waterproofs and strengthens the walls, preventing the vessel collapsing under the tension of the transpiration stream and stopping water leaking out.

Q2. State two factors that increase the rate of transpiration and explain one of them. [3 marks]

• Cue. Increasing light, temperature or air movement (and decreasing humidity) increase the rate; for example wind removes water vapour from around the stomata, keeping the water potential gradient steep so diffusion is faster.

Q3. Name the two pathways by which water crosses the root to the xylem. [2 marks]

• Cue. The apoplast pathway (through the cell walls) and the symplast pathway (through the cytoplasm and plasmodesmata).