The Leaf as the Main Organ for Transpiration

What transpiration is

Water absorbed by the roots travels up through the xylem to the leaves, where most of it evaporates from the moist cell surfaces inside the leaf and diffuses out as water vapour, mainly through the stomata. Only a small fraction of the water taken up is actually used in photosynthesis or retained by the plant.

The continuous column of water inside the xylem vessels is held together by cohesion between water molecules and adhesion to the vessel walls, so as water evaporates from the leaf, it pulls the entire column upward from the roots, an explanation known as the cohesion-tension theory.

Factors affecting the rate of transpiration

Why transpiration matters

These effects are linked: the same evaporation that drives transpiration pull also cools the leaf and keeps its cells turgid, so a plant experiencing very fast transpiration on a hot day may wilt if water loss from the leaves outpaces water uptake by the roots.

• It creates transpiration pull, the main force drawing water and dissolved mineral salts up through the xylem from the roots.
• It helps cool the plant, as evaporating water absorbs heat energy from the leaf.
• It keeps cells turgid, which helps support non-woody parts of the plant.
• It supplies a continuous stream of dissolved mineral ions, such as nitrate and magnesium, to the leaf cells that need them for growth and for making chlorophyll.

How it is examined

Common tasks include describing how a potometer measures the rate of water uptake as an estimate of transpiration rate, explaining why transpiration is faster under certain conditions, or comparing transpiration rates on the upper versus lower leaf surface using cobalt chloride paper.

Worked exam-style question

Question. A potometer is set up with a leafy shoot, and the distance an air bubble moves along the capillary tube in five minutes is recorded under four conditions: still air, a fan blowing across the shoot, the shoot in a plastic bag, and the shoot moved closer to a bright lamp. (a) State which condition would give the fastest bubble movement, and explain why.

(b) State which condition would give the slowest bubble movement, and explain why. (c) Explain why the potometer measures water uptake rather than transpiration directly, and why this is still a useful estimate.

(d) Suggest one precaution needed when setting up the potometer to obtain a reliable reading.

Model answer. (a) The bright lamp gives the fastest bubble movement, because higher light intensity causes more stomata to open wider, increasing the water vapour concentration gradient out of the leaf and so the rate of transpiration. (b) The plastic bag gives the slowest movement, because it traps water vapour around the leaf, raising the humidity immediately outside the stomata and reducing the concentration gradient that drives diffusion outward.

(c) The potometer measures the rate the shoot takes up water, not the rate water vapour leaves the leaf directly, but because most water taken up is lost through transpiration, the rate of water uptake gives a close, workable estimate of the transpiration rate. (d) The apparatus must be assembled under water and all joints sealed (for example with petroleum jelly) so that no air bubbles or leaks enter the system, which would give an inaccurate reading.

Practice question

Try this. A student compares the number of stomata on the upper and lower surfaces of the same leaf using clear nail varnish peels, and finds far more stomata on the lower surface. Explain how this observation relates to the leaf's rate of water loss, and suggest one advantage to the plant of having most of its stomata on the lower surface.

Exam tip

Key terms

Revise these linked terms to answer transpiration questions precisely:

• Transpiration, the loss of water vapour from the plant, mainly through the leaves.
• Transpiration stream, the continuous upward flow of water and mineral salts driven by transpiration.
• Xylem, the tissue that carries water from the roots up to the leaves.
• Stoma, the pore through which most water vapour leaves the leaf.
• Root hair cell, where water absorption from the soil begins the pathway that ends in transpiration.

Source:SRC-DSKP-EN

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