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The rate of photosynthesis depends upon several factors they may be external or internal. Some of the external factors are intensity and duration of sunlight, temperature, availability of CO2, availability of water etc. Even though there are a number of different factors in a physiological process. The rate of the photosynthesis is determined by the limiting or smallest factor. This smallest factor is known as the limiting factor. This law proposed by F.F. Black man is known as the law of limiting factors.

  1. Intensity, quality and duration of light: When sufficient light intensity is obtained it starts photosynthesis. As the intensity of light increases the rate of photosynthesis also increases. Further increase in the light intensity does not increase the rate of photosynthesis; it leads to the destruction of chlorophyll. The rate of photosynthesis is maximum in to occur in the red light. The wave length between 400nm to 700nm is most effective for photosynthesis. Since this is called as photo synthetically active radiation. The long duration of light have a positive effect on the rate of photosynthesis.
  2. Availability of water: It is an essential component of photosynthesis. When there is decrease in the amount of water it results in the closure of stomata. It also results in the decrease of carbon dioxide which also decreases the rate of photosynthesis. Decrease in the amount of water also affects the enzyme activity.
  3. Availability of carbon dioxide: The rate of photosynthesis increase with increase in the concentration of carbon dioxide. High concentration of carbon dioxide causes decrease in the rate of photosynthesis. CO2 is known to be major limiting factor in photosynthesis.
  4. Temperature: The rate of photosynthesis increase with increase in temperature up to a limit. The dark reaction gets harmed adversely by the increase in the temperature. Due to inactivation of the enzyme at high temperature. Whereas the light reaction is less affected. The optimum temperature is different for different plants.
  5. Essential elements: All the essential elements are also having direct effects on the rate of photosynthesis. Nitrogen is basic constituent of chlorophyll.