Factors affecting photosynthesis:
(i) External Factors:
(1) Light: The main source of stored energy in the molecules of carbohydrate by photosynthesis is light. Leaves absorb 80 to 85% of the light falling on them. Only 0.5 to 3.5 % of absorbed light is used for photosynthesis. Rest is wasted in the form of heat or used in transpiration. Light affects the rate of photosynthesis indirectly. Low intensity of light causes stomatal closure, which restricts the entry of CO2 and photosynthetic rate decreases. High light intensity increases the rate of transpiration. This has an inhibitory effect on the rate of photosynthesis.
(2) Oxygen: Photosynthesis rate is reduced due to the increase in the concentration of oxygen in the intercellular space or in the environment.
(3) Carbon dioxide: The concentration of carbon dioxide in the environment is 0.035% (350 ppm), which is often a limiting factor. When the concentration of carbon dioxide increases to 1%, the rate of photosynthesis also increases.
(4) Temperature: Light reaction is not affected by temperature. But dark reaction completely depends on temperature. The rate of photosynthesis is doubled with an increase of every 10°C between 5°C and 30° C.
(5) Water: Water is a compulsory factor for photosynthesis. Only 1% of water absorbed by plants is used for photosynthesis. However, the deficiency of water affects the rate of photosynthesis. The decrease of H2O content of leaves may cause complete closure of the stomatal opening. Another indirect effect of water deficiency is the accumulation of sugar in the cells which increases the rate of respiration and thus, decreases the rate of photosynthesis apparently.
(6) Air pollutants: Sulphur dioxide (SO2) and oxides of nitrogen decrease the rate of photosynthesis.
(ii) Internal Factors:
(1) Chlorophyll contents: It is an important plastid, because it entraps light energy for photosynthesis.
(2) Protoplasmic factor: An unknown factor present in protoplasm affects photosynthesis, which is called the time factor.
(3) Accumulation of carbohydrates: If products of photosynthesis are not transported from leaves to other parts, these products begin to accumulate in photosynthetic cells. As a result of which, the rate of photosynthesis decreases.
