Transpiration PDF

# Transpiration Transpiration is the loss of water vapor from the parts of a plant such as leaves and stems. Water moves in a plant through transpiration stream. The transpiration stream refers to a continuous movement of water through a plant, from the roots to the leaves. When water is lost from...

# Transpiration Transpiration is the loss of water vapor from the parts of a plant such as leaves and stems. Water moves in a plant through transpiration stream. The transpiration stream refers to a continuous movement of water through a plant, from the roots to the leaves. When water is lost from the leaves, more water is drawn up the xylem to replace it. ## Types of Transpiration - **Stomatal transpiration:** water vapor is lost through the stomata on the leaves. When stomata are open, water near the leaf surface changes into vapor and evaporates. Stomatal transpiration is responsible for 85% - 90% of the total water loss in plants. - **Lenticular Transpiration:** Lenticles are small openings in the bark of woody stems. Water evaporates through these lenticels. Water loss by lenticular transpiration is very low. - **Cuticular Transpiration:** The cuticle (a waxy covering on leaf surfaces) allows some water to evaporate. About 5-10% of water is lost this way, however, more water is transpired through the cuticles during dry periods when stomata are closed. ## Factors affecting transpiration ### Environmental factors - **Light:** when light intensity increases, the stomata opens and allows more rapid evaporation. Low light intensity reduces transpiration. - **Wind speed/air movement:** still air lowers the rate of transpiration because water vapor accumulates around the leaves and stems and reduces the rate of diffusion. As wind decreases speed increases, the transpiration rate increases. Water molecules that diffuse out of stomata are quickly blown away from the leaf, this creates a concentration gradient and more water vapor diffuses out of the leaf. - **Humidity:** high humidity decreases transpiration rate because humid air contains a high amount of water vapor, this reduces the diffusion gradient between the inside of the leaf and the outside air, so less water vapor diffuses out of the leaf. - **Low humidity:** increases temperature transpiration rate because dry air contains a low amount of water vapor thus increasing diffusion gradient between the inside of the leaf and the outside air therefore more water vapor diffuses out of the leaf. - **Temperature:** firstly, warmer temperatures water molecules move faster and the rate of evaporation from stomata is there for much faster. Secondly, the water-holding capacity of warm air is greater than that of cold air. Evaporation and transpiration will take place more rapidly in warm air while colder temperatures usually lead to very little or no transpiration. ### Cellular factors. - **Size of leaves:** larger leaves will experience a higher transpiration rate than smaller leaves because wide leaves have more stomata. - **The number of stomata:** the more the number of stomata, the more transpiration occurs. - **Presence of cuticles:** the thicker the cuticle layer on a leaf surface, the slower the transpiration occurs. ## Importance of Transpiration in plants - **Water transport:** transpiration moves water from the roots to other parts of the plants. As water evaporates from the leaf surfaces, this creates a concentration gradient that pulls water upwards through the xylem vessels. - **Temperature regulation:** transpiration cools the plants by releasing heat during water vaporization. The use of heat to convert water into water vapor helps to cool the plant down. - **Nutrient uptake:** nutrients are absorbed by the roots and transported upward along with water.