Transpiration is a major physiological process in plants, by which loss of water from stomata openings takes place. Stoma (singular stomata) are minute pores present in the foliage, stem, roots and flowers. In most of the plant species, the number of stomata is higher in the lower side of the leaf. They are crucial structures for exchanging gases and moisture between plants and their environment. As far as the process of transpiration in plants is concerned, it is similar to evaporation, which most of us are familiar with.
What is Transpiration in Plants?
Apart from loss of water to the environment, several functions of the plant are associated with transpiration process. When you study the detailed structure of a stomata, you will find the guard cells that regulate the opening and closing of stomata. Definitely, transpiration occurs when the stomata are opened. So, what controls the stomatal opening and closing? The hydrostatic pressure of upper plant parts and soil moisture collectively play a major role in opening of stomata.
When there is presence of water in soil, the roots take up the water molecules from the soil by means of osmosis, during which essential plant nutrients are also absorbed. The water and mineral are passed to the upper plant parts. As the stoma receive water molecules, the guard cells become flaccid, resulting in opening of the pores. Water from the plant is then lost by transpiration (diffusion method). After losing water, the cells come to normal state and stomatal openings are closed. This is how, transpiration in plants takes place.
Importance of Transpiration in Plants
As transpiration results in water loss, most people have a query, 'whether transpiration is useful for plants or not.' Botanical studies have found that transpiration is important for survival of the plant. It is crucial for uptake of nutrients from the soil medium and controlling photosynthesis (most importantly for exchange of oxygen and carbon dioxide). In short, plants transpire and lose water for taking up carbon dioxide from the atmosphere.
Transpiration is beneficial for maintaining the plant temperature and keeping the plants cool in hot conditions. The rate of transpiration in plants and number of stomata are proportional to each other. The amount of water loss by transpiration depends on atmospheric temperature, light intensity, humidity, wind, soil water and size of the plant. It is claimed that maximum quantity of water absorbed by the roots is lost through transpiration. Only 1 percent of the absorbed water is utilized for growth and development of plants.
As we all know, cactus and other alike desert plants possess certain adaptive features, which enable them to thrive in prolonged dry spells. One of the adaptations is presence of less number of stomata, sunken stomata and small stomatal structures (in some species). Whichever is the case, the loss of water by transpiration process is greatly reduced. If this has not been the case, it is quite likely that desert plants cannot survive in the dry soil and high atmospheric temperature conditions.
Last but not the least, transpiration and evaporation are important aspects in maintaining the water cycle. With respect to recycling of water, the process of evaporation and transpiration is collectively called evapotranspiration. Approximately 10 percent of the atmospheric moisture is contributed by transpiration process. In biology subject, transpiration in plants is studied in detail. Laboratory demonstrations are kept for easy understanding of the process. Students often opt for transpiration in plants experiment for science fair projects.