Factors Affecting Transpiration
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Direct answer
Transpiration rises when light opens the stomata and when dry, moving air steepens the vapour gradient between the leaf's wet interior and the atmosphere; it falls when humidity is high or soil water runs short. Light is the strongest external factor because stomata open in its presence while it simultaneously warms the leaf. Temperature increases the rate both by raising the saturation vapour pressure inside the leaf and by lowering atmospheric relative humidity, though extreme heat eventually forces stomatal closure. Wind sweeps away the humid boundary layer hugging the leaf surface, so moderately moving air raises transpiration; a hot, dry wind can instead desiccate the leaf until stomata shut. Internal factors — leaf area, stomatal number and distribution, cuticle thickness and root–shoot ratio — set the plant's own ceiling, since 95 to 99 per cent of absorbed water is lost through transpiration rather than used.
What you must remember
- Light: opens stomata (guard-cell turgor rises in light) and heats the leaf — the single most effective external factor.
- Humidity: inversely related; high atmospheric humidity flattens the water-vapour gradient between leaf intercellular spaces and outside air, slowing diffusion out of the stomatal pore.
- Temperature: raises the evaporative capacity of air roughly steeply with each degree; very high temperatures cause water stress and stomatal closure, so the curve is not endless.
- Wind: removes the still, moist boundary layer around the leaf, increasing transpiration; calm air lets that layer build and brake water loss.
- Soil water and root absorption: when roots cannot replace losses, stomata close; a wilted plant stops transpiring almost regardless of the weather.
- Internal factors: leaf area and orientation, number and distribution of stomata (more on the lower surface of dorsiventral leaves), thick cuticle and sunken stomata in xerophytes, and a high root–shoot ratio that supports greater water flow.
- Proportion: about 95–99 per cent of water absorbed by roots escapes as transpiration; the tiny remainder is used in photosynthesis and growth.
A day in the life of one leaf
Follow a dorsiventral leaf from dawn to late afternoon. Sunrise opens the stomata, and transpiration climbs steeply; by late morning, bright light and rising temperature have pushed the rate toward its daily peak. If a breeze picks up at noon, the boundary layer of saturated air coating the leaf is torn away, and the vapour gradient — the engine of the whole process — steepens further. Should the roots now fail to keep pace, the leaf wilts: guard cells lose turgor, the pores close, and transpiration collapses even though the sun still shines. That interplay between the atmosphere's demand and the plant's supply is the practical version of every factor listed above, and it is why midday irrigation of a wilted field can shock roots without helping the leaf.
The same physics explains routine horticulture. Nursery workers mist cuttings to raise the humidity around them, deliberately flattening the vapour gradient so a rootless stem cannot dry out before it roots. Xerophytes arrive pre-adapted: sunken stomata in pits, thick cuticles and reduced leaf area all thicken the effective boundary layer or narrow the pores. And the upward transpiration stream is not waste — the transpiration pull it generates is what lifts mineral-rich water to the top of tall trees in the cohesion–tension mechanism, so a factor that speeds transpiration also speeds ascent of sap.
Where students slip
Direction errors dominate: humidity is inversely related to transpiration while light is directly related, and reversed arrows in a statement question cost easy marks. The second slip is the wind item — moving air increases transpiration by removing the boundary layer, not by "pushing water out", and in exam terms wind's effect is essentially always to raise the rate unless the wording specifies heat-stress closure. Third, remember stomatal transpiration dwarfs cuticular transpiration in most plants, so factors working through the stomata dominate; a question about a xerophyte expects you to invoke sunken stomata and thick cuticle as the internal brake.
Frequently asked questions
Which external factor most strongly influences transpiration?
Light, because it opens the stomata and simultaneously warms the leaf.
How does atmospheric humidity affect transpiration?
Inversely — high humidity reduces the vapour-pressure gradient between the leaf interior and the outside air, slowing the rate.
Why does wind increase transpiration?
It removes the humid boundary layer around the leaf, steepening the gradient for water-vapour diffusion through the stomata.
What internal features reduce transpiration in xerophytes?
Sunken stomata, a thick waxy cuticle, reduced leaf area and fewer stomata, often concentrated on the lower surface.
How much of the water absorbed by roots is lost as transpiration?
Roughly 95 to 99 per cent; only a small fraction is used in photosynthesis and growth.