Water uptake and transpiration rate

1. Water is pulled up a plant because water is being lost from the leaves, so anything that changes the rate of loss changes the rate of uptake at the roots.
2. Over ninety-nine parts in a hundred of the water a plant takes in is lost again as vapour, and only a trace is used in photosynthesis.
3. Water uptake is therefore treated as a good measure of the transpiration rate, although the two are not exactly equal.
4. Every environmental factor works through one of two routes: it changes how fast water evaporates inside the leaf, or it changes how steep the concentration gradient of water vapour is between the leaf and the air outside.
5. A third route exists only for light, which also changes how many stomata are open and so how much escape route there is.
6. The rate is calculated as a volume of water taken up divided by the time taken, so $$rate=\frac{volume\; of\; water\; taken\; up}{time}$$
7. Rates are usually given in ${cm}^3$ per minute or ${cm}^3$ per hour, and a rate can also be worked out from a distance moved along a scale of known cross-sectional area.

Light intensity

1. Increasing the light intensity increases the rate of water uptake.
2. Light causes the guard cells to become turgid, so the stomata open.
3. Open stomata give a larger total area for water vapour to diffuse out through, so more escapes each second.
4. In the dark the stomata close, so transpiration falls to a trickle through the waxy cuticle and water uptake almost stops.
5. Brighter light is usually warmer light as well, so a lamp placed close to a shoot raises the temperature at the same time and the two effects add together.
6. Once every stoma is fully open, raising the light intensity further has little extra effect, so a graph of rate against light intensity levels off.

Air movement

1. Increasing the air movement increases the rate of water uptake.
2. In still air the water vapour leaving the stomata collects just outside them as a layer of damp air.
3. That damp layer makes the concentration of water vapour outside the leaf almost as high as inside it, so the gradient becomes shallow and diffusion slows.
4. Moving air blows that layer away and replaces it with drier air.
5. The gradient is therefore kept steep, and water vapour keeps diffusing out quickly.
6. A gentle breeze produces a large increase, while making an already windy day windier produces much less extra effect, because the damp layer has already gone.
7. In very strong wind some plants close their stomata to protect themselves from drying out, and the rate then falls again.

Temperature

1. Increasing the temperature increases the rate of water uptake.
2. Warmer water molecules have more kinetic energy, so more of them have enough energy to escape from the wet cell walls into the air spaces.
3. Evaporation inside the leaf therefore speeds up and the air spaces hold more water vapour.
4. That raises the concentration inside the leaf, which steepens the gradient out through the stomata.
5. Warmer molecules also diffuse faster once they are in the air, so they move away from the pore more quickly.
6. All three effects push in the same direction, which is why temperature usually has the largest effect of any single factor.
7. At very high temperatures a plant short of water closes its stomata, and the rate then falls even though it is hot.

Humidity

1. Increasing the humidity decreases the rate of water uptake, which makes it the one factor here that works in the opposite direction.
2. Humid air already holds a great deal of water vapour, so the concentration outside the leaf is close to the concentration inside it.
3. The gradient across the stomata is therefore shallow, and water vapour diffuses out slowly.
4. In dry air the outside concentration is very low, the gradient is steep and vapour leaves quickly.
5. Covering a plant with a clear plastic bag traps the vapour it releases, raises the humidity around it and slows its water loss, which is why a cutting is kept in a bag while it grows roots.

When uptake cannot keep up

1. On a hot, bright, windy day water can be lost from the leaves faster than the roots can pull it out of the soil.
2. The cells then lose water, the pressure inside them falls and they stop pressing outwards against their walls.
3. A soft stem is held upright by that pressure, so once it is lost the plant droops and wilts.
4. Wilting reduces further loss, because the drooping leaves hang together and shade one another and the stomata close.
5. Closing the stomata also stops carbon dioxide entering, so photosynthesis and growth stop while the plant is wilted.
6. A wilted plant recovers when it is watered, unless it has been left so long that the cells are damaged beyond repair.