Transport in Plants
Senior 2 · Biology
Transport in Plants
Lesson 4 of 5
24 min
Let's begin
A large tree can lose hundreds of litres of water to the air on a sunny day. That water vapour cools the tree and the surrounding land, and forests such as Nyungwe help keep the air moist and bring rain. For a bean farmer in the dry season, though, the same water loss can mean a wilted crop. Plants in different habitats have evolved very different ways of managing it.
Transpiration is the loss of water vapour from the parts of a plant above the ground, mainly through the stomata of the leaves. Up to 90% of the water taken up by the roots may be lost this way. There are three routes:
Why it matters: transpiration pulls water and dissolved minerals up the plant, cools the leaves as water evaporates, and keeps cells full of water (turgid) so soft parts stay upright. Its cost is water loss, which can cause wilting.
A potometer measures how fast a leafy shoot takes up water. The shoot is cut under water (to keep air out of the xylem) and fitted, airtight, to a water-filled capillary tube containing an air bubble. As the shoot takes up water, the bubble moves along a scale.
Worked example
The bubble moves 45 mm in 5 minutes. Rate = 45 ÷ 5 = 9 mm per minute. If the tube's cross-sectional area is 0.5 mm², the volume taken up = 45 × 0.5 = 22.5 mm³ in 5 minutes, or 4.5 mm³ per minute.
Strictly, a potometer measures water uptake. This is very close to the rate of transpiration, because only a tiny fraction of the water is kept for photosynthesis and growth.
To compare the two surfaces of a leaf, press dry blue cobalt chloride paper against each side. It turns pink with water; the lower surface usually turns pink first, because it has more stomata.
| Factor | Effect of an increase | Reason |
|---|---|---|
| Light intensity | rate increases | stomata open wider in light |
| Temperature | rate increases | water evaporates and diffuses faster |
| Humidity | rate decreases | moist air outside means a smaller difference in water vapour concentration between the leaf and the air |
| Wind speed | rate increases | moving air carries away the moist air around the leaf, keeping the gradient steep |
| Soil water shortage | rate decreases | the plant closes its stomata to save water |
| Group | Habitat | Adaptations |
|---|---|---|
| Mesophytes | moderate water supply; most crops such as beans, maize and cassava | broad leaves with stomata mainly on the lower surface; a moderate cuticle; close their stomata and wilt when water runs short |
| Xerophytes | dry places, such as deserts, rocky ground and the drier parts of eastern Rwanda | thick waxy cuticle; fewer stomata, often sunken in pits or surrounded by hairs that trap moist air; leaves rolled, small or reduced to spines; fleshy (succulent) stems or leaves that store water; deep or widespread roots |
| Hydrophytes | in or on water, such as water lilies, and papyrus in the wetlands along the Nyabarongo and Akagera rivers | stomata on the upper surface of floating leaves; thin or no cuticle; large air spaces (aerenchyma) that keep leaves afloat and carry oxygen to the roots; little xylem and small roots |
| Halophytes | salty soils, such as sea coasts and salt marshes; mangroves are an example | store salts in their cells so their sap is more concentrated than the salty soil water, allowing uptake by osmosis; some get rid of salt through salt glands; fleshy leaves; many xerophyte features; mangroves grow breathing roots (pneumatophores) up out of the airless mud |
Worked example
Explain how sunken stomata surrounded by hairs reduce water loss. Reasoning: the pit and the hairs trap a layer of still, moist air outside the stoma. The water vapour gradient between the inside of the leaf and this trapped air is small, so less water diffuses out, just as transpiration slows on a humid day.
Tip
Four factors, one odd one out: Light, Temperature, Wind all speed transpiration up; Humidity slows it down.
For any xerophyte feature, explain it the same way: it traps moist air, reduces the surface area, thickens the barrier, or stores water.
In a potometer the bubble moved 30 mm in 6 minutes in still air, and 30 mm in 3 minutes with a fan blowing. Calculate both rates and explain the difference.
Solution: Still air: 30 ÷ 6 = 5 mm per minute. With the fan: 30 ÷ 3 = 10 mm per minute. The moving air removes the moist air around the leaves, keeping the water vapour gradient steep, so transpiration doubles.
Why is the shoot cut under water when setting up a potometer?
Solution: So that no air enters the xylem. An air bubble would break the water column and stop the uptake of water.
Give two benefits of transpiration to a plant.
Solution: It draws water and mineral ions up from the roots; it cools the leaves as water evaporates. (It also keeps cells turgid.)
Why do water lilies have their stomata on the upper surface of the leaf?
Solution: The lower surface lies on the water, where gases from the air cannot reach it. Stomata on the upper surface are in contact with the air, so carbon dioxide can enter.
Remember
Sources and credits
