What Limits Photosynthesis? The Limiting Factors

Photosynthesis needs several things at once: light, carbon dioxide, water, chlorophyll and a suitable temperature for its enzymes. If any one of them is in short supply, the whole process slows down, however much there is of the others. The factor that is holding the rate back is called the limiting factor.

Think of a sandwich production line with plenty of bread and fillings but only one person spreading butter. Adding more bread will not make sandwiches any faster; only more butter-spreading will. In the same way, more light does nothing for a plant whose real shortage is carbon dioxide.

In experiments, the rate of photosynthesis is usually measured as the volume of oxygen given off, or the number of oxygen bubbles from pondweed, in a set time. Plot the rate against light intensity and the graph has two parts:

1. A rising slope. Here light is the limiting factor: more light gives a faster rate.
2. A flat plateau. Now more light makes no difference. Something else, such as carbon dioxide concentration or temperature, has become limiting.

If you repeat the experiment with more carbon dioxide or a warmer temperature, the plateau is higher. That tells you which factor was limiting before.

Light and carbon dioxide graphs rise and then level off. A temperature graph rises to a peak and then falls. Warmth gives molecules more energy, so the enzymes of photosynthesis work faster up to an optimum temperature. Above it, the enzymes start to change shape (they are denatured) and the rate drops quickly. This is why grass grows very slowly in a cold winter even on a bright day.

A leaf with less chlorophyll absorbs less light. Chlorophyll is built around a magnesium atom, so a plant short of magnesium makes less of it, and its older leaves turn yellow between the veins.

Water is a reactant, but water shortage usually limits photosynthesis in a less direct way. A plant that is losing too much water closes its stomata, and closed stomata also stop carbon dioxide getting in. That is why photosynthesis slows on hot, dry afternoons and in deserts.

In the classroom pondweed practical, light intensity is changed by moving a lamp. Light spreads out as it travels, so its intensity is inversely proportional to the square of the distance:

light intensity ∝ 1 / distance²

• Lamp moved from 10 cm to 20 cm: the distance doubles, so the intensity falls to 1 ÷ 2² = ¼ of what it was.
• Lamp moved from 30 cm to 10 cm: the distance is divided by 3, so the intensity rises 3² = 9 times.

This is why a small change in distance can make a large change in the bubble count, as long as light is still the limiting factor.

Commercial growers use the same ideas in greenhouses. They can add heat, artificial light and extra carbon dioxide, but each costs money, so they only pay to raise the factor that is actually limiting. Outdoor air is only about 0.04% carbon dioxide, and a crop in a closed greenhouse on a sunny day can use it up faster than it is replaced. Advice for growers from Ontario's agriculture ministry says that raising carbon dioxide to about 1,000 parts per million can increase photosynthesis by about 50%, provided there is enough light.