6.12 - 6.13 - Water uptake and rates
Explain how light, temperature and air movement change transpiration and water uptake. Calculate rates from changes in water mass or volume, with the units and time interval matched correctly.
6.12 - From evaporation to water uptake
Recall that xylem carries water from the roots to leaves, and stomata are pores whose opening is controlled by guard cells. This lesson uses those structures to explain changes in water uptake.
Transpiration
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly from leaves through the stomata.
The route begins inside the leaf and ends in the surrounding air:
- Water delivered by the xylem reaches the moist surfaces of mesophyll cells.
- Liquid water evaporates from those surfaces into the leaf's air spaces.
- Water vapour diffuses out through open stomata, down a water-vapour concentration gradient.
- This loss helps pull more water through the xylem. Roots take up replacement water, so a faster transpiration rate usually causes a faster rate of water uptake by the plant.
Water uptake and transpiration are linked, but they are not two names for the same measurement. Uptake is water entering the plant; transpiration is water vapour leaving its aerial parts. They may not be exactly equal over a short interval because some absorbed water remains in plant tissues or is used in growth and chemical reactions.
Follow the direction: liquid water arrives in xylem, evaporates inside the leaf, and water vapour diffuses out through stomata; replacement uptake follows the loss.
6.12 - Why the environment changes water uptake
Each factor below is an all-else-equal relationship for a well-watered plant over an ordinary range. The factor first changes stomatal opening, evaporation or the water-vapour gradient; transpiration then changes, followed by the demand for replacement water.
| Increased factor | Immediate effect at the leaf | Effect on transpiration and water uptake |
|---|---|---|
| Light intensity | Stomata generally open wider in light so carbon dioxide can enter for photosynthesis. A wider pore also provides an easier route for water vapour to leave. | Transpiration increases, so water uptake usually increases. |
| Temperature | Water molecules have more kinetic energy, so evaporation from mesophyll surfaces is faster and water vapour moves away more rapidly. | Transpiration increases, so water uptake usually increases. |
| Air movement | Moving air removes the moist layer of air beside the leaf. This maintains a steeper water-vapour concentration gradient from inside to outside. | Water vapour diffuses out faster; transpiration and water uptake increase. |
Decreasing any one of these factors usually gives the reverse trend when the others stay constant. The relationships are not limitless: if water loss becomes faster than replacement, guard cells can close stomata and the plant may wilt. That is why a careful explanation says what conditions are being compared instead of claiming that a rate must always rise.
Continue this lesson.
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