Factors that affect the rate of photosynthesis

Photosynthesis is affected by many factors, both external (in the environmental) and internal (inside the plant). External factors include light intensity, the wavelength of light, carbon dioxide levels, temperature, wind velocity, and water and mineral supplies. Internal factors include type and concentration of photosynthetic pigments, enzyme and water content, and leaf structure, and position.

The effect of many of these factors is difficult to determine quantitatively because they interact, they also affect other processes in the plant. For example, the importance of water to photosynthesis cannot be demonstrated easily. Simply depriving a plant of water kills it, but the cause of death may not be connected with photosynthesis. The importance of water can be demonstrated using water labeled with a heavy isotope of oxygen, ¹⁸O, and tracing the isotope using an instrument called a mass spectrometer which can measure the masses of atoms. One batch of Chlorella (green algae) is placed in water in which the oxygen atoms have been replaced by the heavy isotope. Then a second batch of Chlorella in unlabelled water is given a supply of carbon dioxide labeled with ¹⁸O. Only the first batch of Chlorella gives off oxygen labeled with ¹⁸O, confirming that the oxygen formed in photosynthesis comes only from water, not from carbon dioxide.

Light intensity, carbon dioxide concentration, and temperature are three external factors that are relatively easy to manipulate. Consequently they have been the focus of many investigations on photosynthesis.