Factors Limiting the Rate of Photosynthesis
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. In brief: as light intensity rises, the rate of photosynthesis rises too, until another factor, such as carbon dioxide or temperature, becomes limiting and the curve flattens. Photosynthesis and respiration are linked: at the compensation point, the two processes occur at the same rate and there is no net gas exchange.
Growers and ecologists need to know what limits plant growth. This guide explains the idea of a limiting factor, how each of the main factors affects the rate, how to interpret graphs of the rate, how to measure the rate with a pondweed experiment, a worked example with calculations and the common mistakes.
What is a limiting factor?
A limiting factor is a variable that restricts the rate of a process when it is at a low level. Increasing the limiting factor increases the rate. Increasing a factor that is not limiting has no effect. The idea comes from Frederick Blackman, who proposed in 1905 that the rate of a process depending on several factors is limited by the factor nearest its minimum level.
For photosynthesis, the main factors are light intensity, carbon dioxide concentration and temperature. Water and chlorophyll content are important too, but they are less commonly the limit in school experiments.
How does light intensity affect photosynthesis?
Light provides the energy for the light-dependent reactions. At low light, the rate is proportional to the light intensity: doubling the light doubles the rate, as long as light is the limiting factor. At higher intensities, the rate stops increasing, because the enzymes of the Calvin cycle or the supply of carbon dioxide cannot keep up. Very strong light can even damage the photosynthetic apparatus.
The wavelength of the light also matters. Red and blue light are the most effective, and green light is the least effective, as it is mostly reflected. In water, the intensity of light falls with depth, which limits the depth at which algae and plants can grow.
How does carbon dioxide concentration affect photosynthesis?
Carbon dioxide is a raw material of the Calvin cycle. The concentration in air is about 0.04 percent (roughly 420 parts per million at present). In bright light and at a suitable temperature, this is usually the limiting factor. Increasing the concentration raises the rate until another factor limits it, often at around 0.1 percent for many plants, and very high levels may cause the stomata to close.
This is why commercial growers in greenhouses may add carbon dioxide to the air, often to 0.1 percent or higher, to increase yields.
How does temperature affect photosynthesis?
The Calvin cycle is controlled by enzymes, so temperature affects it. At low temperatures, enzyme-catalyzed reactions are slow, and the rate rises with temperature. Typically the rate doubles for every 10 °C rise over the lower range. Above the optimum, which is about 25 to 35 °C for many temperate plants, the enzymes start to be denatured, and the rate falls rapidly. At high temperatures the stomata may also close to save water, which reduces carbon dioxide intake.
The light-dependent reactions are not strongly affected by temperature, because they depend on light energy and not on enzyme activity. This is one way to tell which stage is limiting in an experiment.
How do you read the graphs?
| Part of the graph | What it shows | What limits the rate |
|---|---|---|
| Rising part | Rate increases with the factor on the x-axis | The factor on the x-axis |
| Plateau (flat part) | Increasing the factor has no effect | Another factor |
| Plateau rises when another factor is increased | Second factor was limiting | The newly increased factor |
| Sharp fall at high temperature | Enzymes denatured | Temperature (too high) |
To identify the limiting factor on a plateau, ask which other factor could be raised to lift it. For example, a graph of rate against light intensity at 0.04 percent carbon dioxide may plateau, and if the carbon dioxide is raised to 0.1 percent, the plateau rises. This shows that carbon dioxide was limiting at the original plateau.
How are photosynthesis and respiration related?
A plant respires all the time and photosynthesizes only when there is light. In dim light, the plant uses the oxygen it produces in photosynthesis for respiration, and uses the carbon dioxide from respiration for photosynthesis. At one light intensity, called the compensation point, the two rates are equal, and there is no net uptake or release of carbon dioxide or oxygen. Below this point, the plant loses more through respiration than it gains and cannot grow. Above it, there is a net gain of glucose, which supports growth.
Worked example: the pondweed experiment
The rate of photosynthesis of an aquatic plant, such as Elodea, can be estimated by counting the bubbles of oxygen given off per minute. A lamp is placed at different distances from the plant in a beaker of water containing sodium hydrogencarbonate (to supply carbon dioxide), at a constant temperature.
| Distance (cm) | Bubbles in 3 min | Bubbles per minute | Light intensity (1 ÷ d², cm⁻²) |
|---|---|---|---|
| 10 | 60 | 20 | 0.0100 |
| 20 | 24 | 8 | 0.0025 |
| 40 | 6 | 2 | 0.000625 |
- Rate: bubbles in 3 minutes divided by 3: 60 ÷ 3 = 20, 24 ÷ 3 = 8, 6 ÷ 3 = 2.
- Light intensity: intensity from a point source is proportional to 1 ÷ d². For 10 cm: 1 ÷ 100 = 0.0100; for 20 cm: 1 ÷ 400 = 0.0025; for 40 cm: 1 ÷ 1 600 = 0.000625.
- Pattern: moving the lamp from 20 cm to 10 cm multiplies the intensity by 4 (0.0100 ÷ 0.0025), but the rate rises by 2.5 (20 ÷ 8). From 40 cm to 20 cm, the intensity increases 4 times, and the rate 4 times (8 ÷ 2).
- Interpretation: at low light, the rate is proportional to intensity, as light is limiting. Between 20 and 10 cm, the increase is smaller than the increase in intensity, which suggests that another factor, such as carbon dioxide or temperature, is beginning to limit.
A lamp also gives off heat, so a heat shield, such as a beaker of water, should be placed between the lamp and the plant to keep the temperature constant. Counting bubbles is an approximate method, since bubble size varies, and measuring the volume of gas collected in a syringe is more accurate. Repeating each distance several times and calculating the mean reduces error.
How are limiting factors used in farming?
Commercial greenhouses control light with artificial lamps, temperature with heating and ventilation, and carbon dioxide with enrichment. The aim is to raise all the factors together to the point where the extra cost is no longer worthwhile. Because of this, the best conditions for the grower are not necessarily the highest values. In the open field, farmers cannot control these factors directly but choose planting dates, spacing and varieties that match local conditions.
Which mistakes are common?
Mistake 1: Saying that a plateau means photosynthesis has stopped
It means the rate is constant, because another factor now limits it.
Mistake 2: Increasing the wrong factor
Adding light does not help if carbon dioxide is limiting. Identify the limiting factor first.
Mistake 3: Ignoring the heat from the lamp
The lamp heats the water. In experiments on light intensity, temperature should be controlled.
Mistake 4: Forgetting respiration in gas measurements
Measured oxygen output is the net result of photosynthesis minus respiration, not the total rate of photosynthesis.