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 stoma are bordered by guard cells (together known as stomata) that open and close the pore. 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, changes cell's osmotic pressure, 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.
This table summarizes the factors that affect the rates of transpiration.
| Feature | How this affects transpiration |
|---|---|
| Number of leaves | More leaves (or spines, or other photosynthesizing organ) will have more stomata on their surface for gaseous exchange. This will result in a greater amount of water loss and an increased surface area for evaporation. |
| Number of stomata | More stomata will provide more pores for transpiration. |
| Presence of plant cuticle | A waxy or reflective cuticle will prevent the heating of the leaf. This reduces the temperature and so the rate of evaporation from the leaf. This is essential for plants that wish to conserve water loss, and is found on many xerophytes. |
| Light supply | Stomata are directly related to the rate of transpiration, and these small pores open especially for photosynthesis. While there are exceptions for this (such as night or "CAM photosynthesis"), in general a light supply will encourage open stomata. |
| Temperature | Temperature affects the rate in three ways:
1) An increased rate of evaporation due to a temperature rise will hasten the loss of water. |
| Relative humidity | A drier external surrounding will make a steeper water potential gradient, and so increase the rates of transpiration. |
| Wind | Water lost from transpiration is often left in a residual layer just beneath the leaf. If left alone, this can reduce the amount of water loss as the water potential gradient from inside to outside the leaf is slightly less, due to the accumulation of water vapour there. If there is wind, this is blown away and the gradient remains higher. |
| Water supply | Less water available means there is less to lose. The lack of supply can also prompt other changes that reduce the rates of transpiration. |
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).
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.