By Sunny Datko
What is Transpiration?
Water in the roots is pulled through the plant by transpiration (loss of water vapor through the stomata of the leaves). Transpiration uses about 90% of the water that enters the plant. The other ten percent is used in photosynthesis and cell growth.
How Does Transpiration Work?
Water moves from the soil into plant roots, up through the stem and into the leaves. The water, warmed by the sun, turns into vapor (evaporates), and passes out through thousands of tiny pores (stomata) mostly on the underside of the leaf surface. 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. A simple analogy to help explain the act of transpiration is that a plant transpiring is like a human sweating. Humans sweat by excreting water through pores to cool down. Similarly, a plant goes through transpiration to help carry nutrients throughout the plant and to maintain structure.
What is the Purpose of Transpiration?
Transpiration serves three essential roles:
- Movement of water and nutrients – Moves minerals up from the root (in the xylem) and sugars (products of photosynthesis) throughout the plant (in the phloem).
- Cooling – 80% of the cooling effect of a shade tree is from the evaporative cooling effects of transpiration. This benefits both plants and humans.
- Turgor pressure – Water maintains the turgor pressure in cells much like air inflates a balloon, giving the non-woody plant parts form. Turgidity is important so the plant can remain stiff and upright and gain a competitive advantage when it comes to light. Turgidity is important for the functioning of the guard cells, which surround the stomata and regulate water loss and carbon dioxide uptake. Turgidity is also the force that pushes roots through the soil.
What Environmental Factors Affect Transpiration?
Environmental factors that affect the rate of transpiration include:
- Light:Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.
- Temperature: Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 86°F (30°C), a leaf may transpire three times as fast as it does at 68°F (20°C).
- Relative Humidity: The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the leaf occurs more rapidly.
- Wind: When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air. Oscillating and wall-mount fans can be used to replicate this indoors.
- Growing Media: A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil or soilless media. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor pressure occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.