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The law of limiting factors was proposed by Blackman in 1905.

According to him, the rate of photosynthesis at any given time depends on the factor which is the lowest.

External factors:

1. Light:

  1. Quality: Light between the wavelength 400nm - 700nm is the most effective. Maximum photosynthesis takes place in red light and minimum in green light.
  2. Intensity: The rate of photosynthesis increases steadily with the increase in light intensity. However, if the light intensity goes beyond a certain point, the chlorophyll molecules undergo photo-oxidation and are destroyed.
  3. Duration: Longer duration of light favours photosynthesis. 10 - 12 hours of light per day, favours optimum photosynthesis.

2. Temperature:

Optimum range of temperature ―――→ 10°C to 35°C.

Rate of photosynthesis increases with increase in temperature upto 40°C.

If the temperature is very high, the rate of photosynthesis decreases due to denaturation of enzymes.

3. Water:

Less availability of water keeps a check on the rate of photosynthesis by closing the stomata thereby affecting the entry of CO2.

However, water rarely acts as a limiting factor as less than 1% of the water absorbed by the plant is used in photosynthesis.

4. Carbon Dioxide:

The rate of photosynthesis increases if the CO2 concentration is increased upto a certain limit.

However, very high concentration of CO2 becomes toxic.

5. Oxygen:

Excess oxygen may have an inhibitory effect on photosynthesis.

Enhanced supply of O2 increases the rate of respiration, thereby decreasing the rate of photosynthesis by utilizing the common intermediates.

Besides, O2 destroys the excited state of the chlorophyll molecule and thus inhibits photosynthesis.

Internal factors:

1. Chlorophyll:

Since chlorophyll is directly involved in trapping light energy, it is expected that the amount of chlorophyll will have a direct relationship with the rate of photosynthesis.

However, in reality it has been shown that no such relationship exists between the amount of chlorophyll and the rate of photosynthesis.

2. Anatomy of leaf:

The rate of photosynthesis is influenced by:

  1. The number and frequency of stomatal distribution
  2. Relative proportions and distributions of palisade and spongy parenchyma
  3. Organization of intracellular spaces.

3. Age of leaf:

Newly formed young leaves show high rate of photosynthesis, which gradually increases as the leaves attain maturity, but declines as the leaves start ageing.

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