Limiting Factors

Plants use light energy to make glucose from carbon dioxide and water. The rate of photosynthesis tells us how much photosynthesis takes place in a given time. Even when a plant has everything it needs, that rate cannot increase without limit: one of the conditions may be holding it back. This is a limiting factor. Light intensity, carbon dioxide concentration and temperature can each limit the rate.

Imagine a plant with plenty of water but very little light. Supplying more carbon dioxide may make little difference because the plant cannot absorb enough light energy to use it. Which factor is limiting depends on the conditions; it is not a permanent property of the plant.

Temperature: an optimum, then a fall

Photosynthesis involves reactions controlled by enzymes. At low temperatures, these reactions are slow. As temperature rises, particles move faster and enzyme-controlled reactions usually happen more quickly, so the rate of photosynthesis increases. It reaches an optimum temperature at which the rate is highest under those conditions.

Above the optimum, high temperatures can denature the enzymes: their shapes change so they no longer work effectively. The rate of photosynthesis then decreases. The optimum is not one fixed temperature for every plant or set of conditions.

Light intensity: more energy, until something else limits the rate

Chlorophyll absorbs the light energy needed for photosynthesis. When light intensity is low, increasing it allows the plant to absorb more energy, so the rate rises. Eventually the graph levels off. At that point, increasing light intensity alone cannot raise the rate further under those conditions; another factor, such as carbon dioxide concentration or temperature, is restricting it.

Carbon dioxide concentration: more raw material, until a plateau

Carbon dioxide is a raw material for photosynthesis. When its concentration is low, supplying more carbon dioxide usually increases the rate because more is available for the reactions. The rise does not continue indefinitely. Once carbon dioxide is no longer the factor restricting the rate, the graph levels off even if its concentration increases further. Light intensity or temperature may then be limiting.

These curves show why a limiting factor must be identified from the conditions, not just from the name of the factor being changed. The temperature curve has a peak and then falls; the light and carbon dioxide curves rise and then level off.