Transpiration

• Trasnpiration is the evaporation of water from a plant's surface, especially the leaves as a result of gas exchange.
• A plant needs to open its stomata to let in carbon dioxide so that it can produce glucose, but this also lets water out. So transpiration is a side effect of the gas exchange needed for photosynthesis.
• Four main factors affect transpiration rate; Light, temperature, humidity and wind.
• Light - the lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis. When it's dark the stomata are usually closed, so there is little transpiration.
• Temperature - The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate forom the cells in the leaf faster. This increases the water potential gradient.
• Humidity - the lower the humidity, the faster transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration.
• Wind - The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases transpiration rate.
• A potometer is a special piece of apparatus usedd to estimate transpiration rates. It actually measures water uptake by a plant, but it's assumed that water uptake by the plant is directly related to water loss by the leaves.
• Xerophytes are plants like cacti and marram grass (which grows on sand dunes). They're adapted to live in dry climates. Their adaptations prevent them losing too much water by transpiration.
• Marram grass has stomata that are sunk in pits, so they're sheltered from the wind. This helps to slow transpiration down.
• Marram grass has a thin layer of 'hairs' on the epidermis - this traps moist air around the stomata, which reduced the water potential gradient between the leaf and the air, slowing transpiration down.
• In hot or windy conditions marram grass plants roll their leaves - again this traps moist air, slowing down transpiration. It also reduces the exposed surface area for losing water and protects the stomata from wind.
• Both marram grass and cacti have a thick, waxy layer on the epidermis - this reduces water loss by evapouration because the layer is waterproof.
• cacti have spines instead of leave - the reduces the surface area for water loss.
• Cacti also close their stomata at the hottest times of the day when transpiration rates are the highest.
• Hydrophytes are plants like water lilies, which live in aquatic habitats. As they grow in water, they don't need adaptions to reduce water loss, but they do need adaptations to help them cope with low oxygen level.
• Air spaces in the tisssues of hydrophytes help the plants to float and can act as a store of oxygen for use in respiration. Floating on the water surface increases the amount of light they recieve.
• Stomata in hydrophytes are usually on present on the upper surface of floating leaves to maximise gas exchange.
• Hydrophytes often have flexible leaves and stems - these plants are supported by the water around them, so this flexibility helps to prevent damage by water currents.