Coaching Centre
Leaves, light and water
Which colours of light chlorophyll and other leaf pigments absorb, the trade-off between taking in carbon dioxide and losing water through stomata, and how Australian leaves, C4 grasses and CAM plants are adapted to shade, heat and drought.
Every time a leaf opens its pores to take in carbon dioxide, water escapes. A big gum tree can lose hundreds of litres of water on a hot day, and almost none of it is used in photosynthesis. For a plant, making food is a constant bargain between getting enough carbon dioxide and not drying out.
Quick recap
In Year 8 (2B) you met the photosynthesis equation and the parts of a leaf:
carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy absorbed by chlorophyll)
This page looks at the two things every leaf has to manage: capturing light and getting carbon dioxide without losing too much water. Different habitats solve these problems in very different ways.
Which light does a leaf use?
Sunlight is a mix of colours. Chloroplasts contain several pigments, each absorbing particular colours:
- Chlorophylls a and b absorb blue-violet and red light most strongly, and absorb green least. Much of the green light is reflected or passes straight through, which is why leaves look green.
- Carotenoids are yellow and orange pigments. They absorb blue-green light and pass the energy to chlorophyll. They also protect the leaf by getting rid of excess light energy that could damage it.
A graph of how much light a pigment absorbs at each colour is its absorption spectrum. A graph of how fast photosynthesis happens at each colour is the action spectrum. The two have the same shape, with peaks in the blue and the red, which is good evidence that chlorophyll is the pigment doing the work. Green light isn't completely wasted, though: it gets deeper into a leaf than red or blue, so lower cells still use some of it.
In autumn, deciduous trees such as maples break down their chlorophyll and pull the nutrients back into their stems. The carotenoids that were there all along show through as yellow and orange, and some trees also make red pigments called anthocyanins. Most Australian native trees are evergreen, but you see anthocyanins in the bright red new growth of many eucalypts, lilly pillies and other natives. The red is thought to act like sunscreen for young leaves while their chloroplasts are still developing.
The stomata trade-off
Carbon dioxide gets into a leaf through stomata, tiny pores mostly on the underside. Each stoma is surrounded by two guard cells. When the guard cells take in water they swell and bend apart, opening the pore. When the plant is short of water they go limp and the pore closes.
An open stoma lets carbon dioxide in, but it also lets water vapour out. This loss of water from leaves is called transpiration. It isn't all bad: evaporation cools the leaf, and it pulls water and dissolved minerals up from the roots through the xylem. But a plant loses hundreds of water molecules for every carbon dioxide molecule it takes in, and roughly 97% of the water its roots absorb ends up back in the air.
So every leaf faces the same choice. Stomata wide open means more carbon dioxide and more sugar, but a risk of wilting. Stomata closed saves water, but photosynthesis slows to a stop and the leaf can overheat because it is no longer being cooled. Plants close their stomata on hot, dry afternoons, which is why many plants make most of their sugar in the morning.
Leaves built for Australia
| Habitat | Main problem | Leaf features |
|---|---|---|
| Rainforest understorey | Very little light reaches the forest floor; water is plentiful | Large, thin, flat leaves held at right angles to the light; thin cuticle; many have a pointed drip tip so rain runs off and the leaf dries quickly |
| Eucalypt woodland | Strong sun, hot summers, poor soils | Adult eucalypt leaves hang vertically, edge-on to the midday sun, so they catch gentler morning and afternoon light and stay cooler; stomata on both sides; thick, waxy cuticle; tough leaves |
| Desert spinifex country | Intense heat and very little water | Spinifex (Triodia) leaves are rolled into tight tubes with the stomata hidden in grooves on the inside, lined with hairs that trap still, moist air |
Many Australian plants are sclerophylls ("hard leaves"): tough, leathery leaves stiffened with extra fibre, with thick cuticles. They suit hot, dry conditions and also Australia's old, nutrient-poor soils, because a tough leaf lasts for years and isn't easily eaten. In hakeas and banksias the stomata sit in sunken pits lined with hairs, which slows the escape of water vapour.
Some plants have changed the chemistry of photosynthesis itself:
- C4 plants, including spinifex, kangaroo grass and crops such as sugarcane and maize, first capture carbon dioxide in one set of cells and pump it into another, where it is concentrated around the enzyme that builds sugar. They can photosynthesise quickly with their stomata only partly open, so they lose less water and keep working in heat that slows other plants down. Most of the grasses of Australia's tropical and arid regions are C4.
- CAM plants, such as cacti, pineapples and some succulents including pigface, open their stomata at night, when it is cool and humid, and store the carbon dioxide as an acid. During the day they keep their stomata shut and use the stored carbon dioxide for photosynthesis.
Aboriginal people have long made use of these plant properties. Resin from spinifex, which helps protect the plant, was heated and worked into a strong glue for fixing stone points to spears and handles, and it is still used today.
- The starting design wilts in eucalypt woodland. Change one thing at a time until it thrives. Which change helped most?
- Switch to the rainforest understorey. Design a leaf that thrives. Why is a thick cuticle a waste of sugar here?
- Take your best woodland design into the desert. What goes wrong? Now find a design that thrives in the desert.
- In each habitat, what is the main limit on the leaf: light, carbon dioxide or heat? Use the stomata trade-off to explain your answer.
Test yourself
Chlorophyll absorbs light most strongly in the
Peaks in blue and red; green is reflected.
On a hot, dry afternoon a plant closes its stomata. The most likely result is that
Closed stomata save water but block carbon dioxide, and the leaf loses evaporative cooling.
Adult eucalypt leaves often hang vertically. This helps them
Edge-on leaves catch gentler morning and afternoon light.
CAM plants such as cacti save water by
They take in CO₂ at night, then use it in daylight with stomata closed.
Label the leaf adaptations.
Which habitat does each leaf feature suit best?
Pores in a leaf are called , and each is opened and closed by two cells. Loss of water vapour from leaves is called . Yellow and orange leaf pigments are . A graph of the rate of photosynthesis at each colour of light is the spectrum.
Explain why a plant cannot have its stomata fully open all the time in a hot, dry environment, and describe two adaptations that help desert plants deal with this problem.
Stomata must open to let carbon dioxide into the leaf for photosynthesis, but water vapour escapes through the same pores. In hot, dry air the water evaporates very quickly, so if the stomata stayed fully open the plant would lose water faster than its roots could take it up, and it would wilt and could die. Spinifex reduces this by rolling its leaves into tubes, with the stomata inside grooves lined with hairs; the still, moist air trapped there slows evaporation while some carbon dioxide can still diffuse in. CAM plants such as cacti open their stomata only at night, when it is cooler and more humid so less water is lost, and store the carbon dioxide to use for photosynthesis during the day with their stomata closed.