Question
A student notices that a well-watered plant wilts on a hot, dry afternoon but recovers by morning. Explain the mechanism of transpiration and how guard cells regulate water loss through stomata.
Solution — Step by Step
Transpiration is the loss of water vapour from aerial parts of the plant — mostly through stomata in leaves. Think of it as the plant “sweating,” except unlike us, the plant uses this process to create a pressure gradient that pulls water all the way up from roots to leaves.
Water molecules stick to each other (cohesion) and to xylem walls (adhesion). When water evaporates from mesophyll cells into air spaces and exits through stomata, it creates a tension in the xylem column — like pulling a chain from the top. This tension is transmitted all the way to the roots, pulling up more water. This is the cohesion-tension theory, and it’s the main mechanism of water ascent in plants.
Guard cells are the two bean-shaped cells surrounding each stomatal pore. Their inner walls (facing the pore) are thicker than their outer walls. When guard cells absorb water and become turgid, they bow outward — the thick inner wall curves, pulling the pore open. When they lose water and become flaccid, the pore closes. This is purely mechanical, driven by turgor pressure.
In daylight, guard cells photosynthesise → K⁺ ions actively pump in → water follows by osmosis → cells become turgid → stomata open. At night (or when the plant is stressed), K⁺ moves out → water leaves → cells become flaccid → stomata close. This is why the wilted plant from the question recovers by morning: stomata close at night, water loss stops, roots keep absorbing, turgor is restored.
Four factors matter most for NEET:
| Factor | Effect on Transpiration |
|---|---|
| Light | Increases (stomata open) |
| Temperature | Increases (faster evaporation) |
| Humidity | Decreases (less vapour pressure gradient) |
| Wind | Increases (removes humid air near leaf) |
Why This Works
The entire system is driven by the vapour pressure gradient between the humid air spaces inside the leaf and the drier outside air. As long as outside air is drier than the inside, water will evaporate. The plant doesn’t “pump” water up — it simply lets physics do the work by maintaining continuous evaporation at the top.
Guard cells are remarkable because they respond to multiple signals simultaneously: light intensity, CO₂ concentration, water stress (via ABA — abscisic acid), and temperature. When the plant is water-stressed, ABA is released, which triggers K⁺ efflux from guard cells, closing the stomata even in daylight. This is why plants wilt on a hot dry day but not just a hot day.
The afternoon wilting in the question happens because transpiration rate exceeds water absorption rate — the soil can supply water, but not fast enough. By morning, temperatures drop, stomata close, and absorption catches up.
Alternative Method — The Potometer Approach
For experimental questions (common in NEET practicals section):
A potometer measures transpiration indirectly by measuring water uptake. An air bubble introduced into the capillary tube moves as the plant absorbs water. The rate of bubble movement = approximate rate of transpiration.
Potometer measures water absorption, not water loss directly. In most conditions these are almost equal, but in high humidity, absorption > transpiration briefly as the plant stores water. This distinction has appeared in NEET MCQs.
Common Mistake
Students write that stomata open because “guard cells become turgid and swell.” Swelling alone isn’t enough — it’s the unequal wall thickness that matters. Both cells swell, but because the inner wall is thicker and inextensible, the outer wall bulges outward, bending the cell into a crescent and pulling the pore open. Without this structural detail, the answer is incomplete for a 3-mark board question.
Also: many students confuse the direction — guard cells becoming turgid → stomata open (daytime). Becoming flaccid → stomata close (night/stress). Mix this up and the whole answer inverts.
Osmotic pressure: Higher solute concentration → lower water potential → water moves in
Transpiration pull: Evaporation at leaf → tension in xylem → water pulled from roots
Guard cell state:
- Turgid (K⁺ in, water in) → Stomata open
- Flaccid (K⁺ out, water out) → Stomata closed
ABA = stress hormone → closes stomata (remember: ABA = Adverse conditions)
This topic carries reliable weightage in NEET — expect 1-2 MCQs on guard cell mechanism, factors affecting transpiration, or the cohesion-tension theory. The afternoon wilting scenario specifically appeared in CBSE board practicals and NEET 2022. Get the guard cell mechanism crisp and you’ve covered most of the marks here.