Transpiration
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
stomata 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) .
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.
*
Antitranspirant*
Ecohydrology*
Evapotranspiration*
Moisture stress*Measuring transpiration with a
potometer*
Transpirational pull*
A description of transpiration, including a short animation illustrating the process
