Transpiration and Stomata

• Transpiration is affected by light intensity - the brighter the light, the greater the transpiration rate. Stomata begin to close as it gets darker. Photosynthesis can't happen in the dark, so they don't need to be open to let CO2 in. When the stomata are closed, very little water can escape.
• Transpiration is affected by temperature - the warmer it is, the faster transpiration happens. When it's warm the water particles have more energy to evaporate and diffuse out of the stomata.
• Transpiration is affected by air flow - the better the air flow around a leaf (e.g. stronger wind), the greater transpiration rate. If air flow around a leaf is poor, the water vapour just surrounds the leaf and doesn't move away. This means there's a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn't happen as quickly. If there's good air flow, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf. Diffusion then happens quickly from an area of higher concentration to an area of lower concentration.
• Transpiration is affected by humidity - the drier the air around a leaf, the faster transpiration happens. This is like what happens with air flow. If the air is humid there's a lot of water in it already, so there's not much of a difference between the inside and the outside of the leaf. Diffusion happens fastest if there's a really high concentration in one place, and a really low concentration in the other.
• You can estimate the rate of transpiration by measuring the uptake of water by a plant. This is because you can assume that water uptake by the plant is directly related to water loss by the leaves (transpiration). You can use a piece of apparatus called a potometer.
• Guard cells have a kidney shape which opens and closes the stomata in a leaf. When the plant has lots of water, the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis.
• When a plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapour escaping.
• Guard cells are sensitive to light and close at night to save water without losing out on photosynthesis.
• You usually find more stomata on the undersides of leaves than on the top. The lower surface is shaded and cooler - so less water is lost through the stomata than if they were on the upper surface.
• Guard cells are adapted for gas exchange and controlling water loss within a leaf.