2.13 - Transpiration

Transpiration and the transpiration stream

• Transpiration is the loss of water vapour from a plant, and it happens mostly at the leaves.
• It follows this sequence inside the leaf:
  1. Water evaporates from the surface of the mesophyll cells inside the leaf, turning into water vapour.
  2. The water vapour collects in the air spaces between those cells.
  3. It diffuses out of the leaf through the stomata (tiny pores in the leaf surface).
• This diffusion happens because there is much more water inside the leaf than in the drier air outside.

Why transpiration happens

• Transpiration is really a side-effect of the way a leaf is built for photosynthesis.
• The leaf must have stomata so carbon dioxide and oxygen can be exchanged with the air.
• Having those pores open also lets water vapour escape - so while the stomata are open for gas exchange, the plant unavoidably loses water by transpiration.

The transpiration stream

• As water is lost by transpiration, it leaves a slight shortage of water in the leaf.
• More water is drawn up to replace it, moving up through the xylem vessels from the rest of the plant.
• This in turn draws more water up from the roots, giving a continuous flow of water from the roots, up through the xylem, and out of the leaves.
• This steady upward flow is called the transpiration stream.
• The transpiration stream also carries dissolved mineral ions up from the roots along with the water, distributing them to the stems and leaves where they're needed.

The structure and function of the stomata

• Stomata are tiny pores in the surface of a leaf, found mostly on the lower surface.
• They let carbon dioxide and oxygen diffuse in and out for photosynthesis, and let water vapour escape during transpiration.
• Because losing too much water is dangerous, the plant can open and close its stomata to control how much water is lost.

How guard cells open and close the pore

• Each stoma is surrounded by a pair of guard cells, which change shape to control the size of the pore.
• Guard cells have thin outer walls and thickened inner walls. This uneven thickening makes them curve open and close as they gain or lose water.
• Turgid (swollen with water) - The guard cells bend apart and the stoma opens.
• Flaccid (short of water and limp) - The guard cells straighten and the stoma closes.

The change in the guard cells is driven by their water content

1. In response to light, potassium ions are pumped into the guard cells.
2. This raises their solute concentration and so lowers their water concentration.
3. Water moves into the guard cells by osmosis, making them turgid, so the stoma opens.
4. When the potassium ions leave the guard cells again, their water concentration rises.
5. Water moves out by osmosis, the guard cells become flaccid, and the stoma closes.

When stomata open and close:

• The stomata close automatically when the plant starts to run short of water.
• Because guard cells are sensitive to light, the stomata tend to open during the day and close at night - saving water without losing out on photosynthesis.

Factors that affect the rate of transpiration

• The rate of transpiration is how quickly a plant loses water.
• Transpiration happens by water vapour diffusing out of the leaf through the stomata.
• Anything that steepens the difference in water concentration between the inside of the leaf and the air outside, or that opens the stomata wider, speeds it up.

The four environmental factors

1. Light intensity - The brighter the light, the faster transpiration. In bright light the stomata open to let carbon dioxide in for photosynthesis, which also lets more water vapour out. As it gets darker the stomata begin to close (photosynthesis can't happen in the dark), and when closed very little water escapes.
2. Temperature - The warmer it is, the faster transpiration. When warm, water molecules have more energy, so they evaporate and diffuse out of the stomata more quickly.
3. Humidity - The drier the air, the faster transpiration. Humid air already holds a lot of water, so there's only a small concentration difference between the inside and outside of the leaf, and diffusion out is slow. Dry air gives a large difference, so water diffuses out quickly.
4. Air movement (wind speed) - The better the air movement, the faster transpiration. In still air the water vapour sits around the leaf, keeping the outside concentration high and diffusion slow. Moving air sweeps the water vapour away, keeping the outside concentration low, so water continues to diffuse out quickly.

Surface area

• The greater the surface area of the leaves, the faster transpiration.
• A larger surface carries more stomata for water vapour to escape from.

Rate of transpiration from a potometer

• A potometer measures how much water a cut shoot takes up. Almost all of that water is lost by transpiration, so the water uptake is taken as a measure of the transpiration rate.
• As the shoot takes up water, an air bubble moves along a scale. The distance the bubble moves in a given time gives the rate.
• The rate is the distance moved ÷ time taken.
• The rate can also be found from the mass a plant loses over time, since transpiration makes the plant lighter - dividing the change in mass by the time gives the rate of water loss.

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Worked example - Rate of transpiration from a potometer

An air bubble in a potometer moves 36 mm in 8 minutes. Calculate the rate of transpiration.