transpiration

Stoma in a tomato leaf shown via colorised scanning electron microscope

The clouds in this image of the Amazon Rainforest are a result of transpiration.

Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants (similar to sweating), especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells that open and close the pore.^[1] Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.

Mass flow of liquid water from the roots to the leaves is driven in part by capillary action. In taller plants and trees however, the force of gravity can only be overcome by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.

Plants regulate the rate of transpiration by the degree of stomatal opening. The rate of transpiration is also influenced by the evaporative demand of the atmosphere surrounding the leaf such as humidity, temperature, wind and incident sunlight. Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Stomatic transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the cuticle of the leaves and young stems. Transpiration serves to evaporatively cool plants as the escaping water vapor carries away heat energy. A plant should not be transplanted in full sunshine because it may lose too much water and wilt before the damaged roots can supply enough water. ^[2]

This table summarizes the factors that affect the rates of transpiration.

Some xerophytes will reduce the surface of their leaves during water deficiencies (left). If temperatures are cool enough and water levels are adequate the leaves expand again (right).

A fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced).^[3]

Transpiration rates of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, photosynthesis systems and heat balance sap flow gauges.

Desert plants and conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis, in which the stomata are closed during the day and open at night when transpiration will be lower.

See also

• Antitranspirant – a substance to prevent transpiration
• Eddy covariance flux (aka eddy correlation, eddy flux)
• Hydrology (agriculture)
• Latent heat flux
• Water Evaluation And Planning system (WEAP)
• Soil plant atmosphere continuum

References

External links

• Transpiration by Trees
• USGS The Water Cycle: Evapotranspiration

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