Transpiration — Notes

Transpiration is the loss of water in the form of vapour from the aerial parts of a plant. Water evaporates from the moist cell walls of the leaf into the air spaces and diffuses out.

Types (by route): stomatal (through stomata; most of the loss), cuticular (through the cuticle; small), lenticular (through lenticels in bark; very little).

Almost all the water absorbed is lost this way (for example, a plant that absorbs 500 mL and keeps 6 mL loses 98.8 per cent).

Importance: creates the transpiration pull that lifts water and dissolved minerals through the xylem; cools the leaves; distributes minerals. Because it also wastes water and can cause wilting, it is called a necessary evil.

A stoma is a pore surrounded by two bean-shaped guard cells, which (unlike other epidermal cells) contain chloroplasts. Their inner walls (towards the pore) are thick; their outer walls are thin.

Opening (light): potassium ions (K+) move from the neighbouring epidermal cells into the guard cells → solute concentration rises → water enters by osmosis → guard cells become turgid → thin outer walls stretch more, the cells curve apart → pore opens.

Closing (dark, or when the plant is short of water): K+ ions move out → water leaves by osmosis → guard cells become flaccid and straight → pore closes.

Loss of weight: two flasks of water, each with a layer of oil (to stop evaporation): A with a leafy shoot, B with a leafless shoot. Weigh at the start and after some hours. A loses much more weight (for example 11.2 g against 0.8 g in 8 hours). Conclusion: water is lost mainly through the leaves. A potted plant with its pot and soil wrapped in polythene can be weighed in the same way.

Bell jar: a watered potted plant with its pot and soil covered in polythene, under a bell jar on a glass plate; control: a covered pot without a plant. Observation: drops of water on the inner wall of the jar with the plant only (they turn dry cobalt chloride paper pink). Conclusion: the plant gives off water vapour.

Cobalt chloride paper on a dorsiventral leaf: dry blue paper on both surfaces, covered with glass slides and clipped. Observation: the paper on the lower surface turns pink first. Conclusion: more water is lost from the lower surface, which has more stomata. (On an isobilateral leaf such as maize both papers change at about the same time.)

A potometer measures the rate of water uptake by a leafy shoot. The shoot is fitted with an airtight cork into a water-filled apparatus; an air bubble in a graduated capillary moves towards the shoot as water is taken up. A reservoir with a stopcock resets the bubble.

Precautions: cut the shoot under water (no air in the xylem); make all joints airtight with Vaseline; keep conditions steady.

Why it approximates transpiration: nearly all the water absorbed is lost by transpiration.

Limitations: it measures uptake, not loss; it uses a cut shoot, not a whole plant; air leaks and temperature changes affect the bubble.

Adaptations to reduce transpiration: leaves reduced to spines (Opuntia); thick waxy cuticle; sunken stomata; fewer stomata, mostly on the lower surface; hairs or rolled leaves; shedding leaves in the dry season.

Guttation: loss of water as liquid drops (containing dissolved salts) through hydathodes at the tips and margins of leaves, at night or early morning when root pressure is high and transpiration is low; for example grasses, tomato, garden nasturtium.

Bleeding: oozing of sap from a cut or injured part of a plant, due to root pressure.