Transpiration

I

Introduction

Transpiration, biological process in which water evaporates from a plant, especially through tiny openings called stomata on the surfaces of leaves.

All living things need continuing supplies of water to survive. A plant needs water to keep up the internal pressure or turgidity in its cells and tissues (which maintains the plant's shape), to bring in dissolved minerals and raw materials from the soil, and for photosynthesis.

II

Water Flow Through a Plant

Water from the soil carrying dissolved minerals enters a plant through the roots and flows along microscopic stiff xylem tubes up the stem or trunk to the leaves. This brings the water and minerals to the places where they are required. To maintain the vital flow, water evaporates as invisible water vapour from the leaves into the air. As water evaporates from the leaves, more comes into the roots to replace it. In fact, water is pulled in a continuous stream through the plant, from root to leaf, by capillary action—a wick or suction effect, known as transpiration tension. Much of the water passes through the plant and into the air, without being taken into the plant's cells.

On a hot day, the leaves of a large oak can lose 200 litres (53 gallons) in an hour by transpiration. One hectare of tropical forest releases 200,000 litres (53,000 gallons) of water vapour daily into the atmosphere. This is 20 times the rate of direct evaporation from a lake or sea. Transpiration returns massive volumes of water from the ground to the atmosphere and is a very important part of the general water cycle on Earth.

III

Control of Transpiration

As water is often in short supply, most plants control transpiration. A leaf's surface layer, the epidermis, has a waxy, waterproof coating, the cuticle, to minimize evaporation through it. Inside the leaf is the parenchyma, a spongy layer of cells and air spaces. Water loss actually occurs here, as water evaporates from cell surfaces into the air spaces, and then passes as water vapour through microscopic leaf pores or stomata in the epidermis to the outside Each stoma is flanked by two specialized guard cells. A guard cell's inner (stoma-side) wall is thicker and stiffer than its outer wall. When a plant begins to wilt through too much transpiration, the guard cells become floppy or flaccid with lack of water. A flaccid guard cell is straight, so the stoma between them is closed to a thin slit. When thousands of stomata shut on a leaf, the rate of transpiration is drastically reduced. The plant continues to absorb water through the roots. This replenishes the plant cells which swell and become turgid. As the guard cells swell, the thin, outer wall stretches more easily than the thick, inner wall so they become curved, making the stoma between them wider and rounder. More water vapour passes through the open stomata, and transpiration increases until the plant begins to wilt again. This self-adjusting system regulates the plant's water loss by transpiration. The concentration of the plant hormone abscisic acid also increases when the leaves of a plant wilt as it causes the guard cells to close the stomata.

Stomata allow gas exchange: water vapour out and atmospheric carbon dioxide in for photosynthesis. Closed stomata reduce transpiration and limit photosynthesis and so a plant must balance these two processes. Most plants have stomata only on the underside leaf surfaces, which lose less water than the sunny upper surfaces. Also, stomata open to allow gas exchange late at night and during the morning, becoming smaller at midday and during the afternoon.

IV

Adaptations

Plants in windy, dry, or hot places have many adaptations to reduce the evaporation that causes transpiration. Some, such as heathers, have a curled leaf shape, with the stomata on the inside, out of the wind. Leaf hairs, as on the pig's-ear plant, work as micro-windbreaks to cut down evaporation. Mountainside plants keep their leaves close to the ground, as mounds or cushions, out of the drying wind.

Desert cacti have extra-thick, waxy cuticles, and only a few sunken stomata. Some open their stomata during the cooler night, to reduce transpiration and let in carbon dioxide, which is then stored in chemical form for use next day. Compared to the same area of typical woodland leaf, a cactus such as the prickly pear loses only one-thirtieth the amount of water.

In winter, water may be unavailable as it is frozen in the soil. Deciduous trees stop transpiration (and photosynthesis) by losing their leaves and sealing the leaf-stalk scars. The needle leaves of the conifer have thick, waxy cuticles, and stomata sunk in deep grooves, to minimize evaporation. This enables the conifer to keep its needle leaves even in winter.