4.2 Limiting factors and the rate of photosynthesis

Limiting factors

The rate of photosynthesis is affected by:

• light intensity
• carbon dioxide concentration
• temperature
• the amount of chlorophyll (leaves with less chlorophyll, for example because of a disease or a lack of magnesium, photosynthesise more slowly).

Reading the graphs

• Light intensity and carbon dioxide: at first the rate rises as the factor increases, so that factor is limiting. Then the graph levels off: increasing it further makes no difference, because another factor is now limiting.
• Temperature: the rate rises as temperature rises, because the enzymes that control photosynthesis work faster. Above the optimum, the enzymes are denatured and the rate falls quickly.

How factors interact (Higher)

On the graph, at low light intensity both lines are the same: light is limiting. Where the lower line levels off, raising the carbon dioxide concentration makes the rate higher, so carbon dioxide was limiting there. Only one factor limits the rate at any moment.

Greenhouses (Higher)

Farmers use greenhouses to give crops the best conditions:

• paraffin heaters keep the temperature up and release carbon dioxide as they burn
• artificial lighting lets photosynthesis continue at night
• the greenhouse can be enriched with carbon dioxide.

The grower must balance the cost of heating and lighting against the extra money from a bigger crop. It is not worth adding more of a factor that is not limiting.