Transpiration

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Transpiration may also refer to sweating and hyperhydrosis.

Transpiration is the evaporation of water from aerial parts of plants, especially leaves but also stems, flowers and fruits. Transpiration is a side effect of the plant needing to open its stomates in order to obtain carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients from roots to shoots. Mass flow is caused 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.

The rate of transpiration is directly related to whether the stomata are open or closed. The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, wind speed, and soil water supply. The reason that an increase in temperature will cause an increase in transpiration rate is because an increase in temperature will cause more water to evaporate from the cell walls inside the leaf. This will increase the water potential gradient between the leaf interior and the outside air causing water to leave the leaf more quickly, thereby increasing the rate of transpiration.

A fully grown tree may lose several hundred gallons (a few cubic meters) 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) (Martin, Leonard & Stamp 1976, p. 81).

Transpiration rate can be measured by a potometer. There are two kinds: mass potometers, which measure transpiration, and bubble potometers, which measure water uptake.

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 cactus 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.