Photosynthesis is influenced or affected by internal as well as external factors of the plant.
Law of limiting factor: If a chemical process is affected by more than one factor, then its rate will be determined by factor which is nearest to its minimal value.
I. External factors
a) Light:
- Incident light is directly proportional to carbon dioxide fixation rate, but at higher light intensities the rate does not increase further as other factors becoming limiting.
- Light is rarely a limiting factor because light saturation occurs at 10% of the full sunlight.
.b) Carbon dioxide concentration: It is a major limiting factor.
- Carbon dioxide usually low in atmosphere(0.03-0.04%) up to 0.05% which increases the rate of carbon dioxide fixation.
- Beyond 0.05% of carbon dioxide the rate of photosynthesis is decreased.
- Both C3 and C4 plant show increase in rate of photosynthesis at high light intensities accompanied by high concentration of carbon dioxide saturation point for C3 is obtained at higher concentration as compared to C4. Hence, carbon dioxide concentration is more of a limiting factor for C3 plants.
- Increased carbon dioxide concentration is beneficial for green house crops such as tomato and bell pepper.
c) Temperature:
- The optimum temperature of C3 plant is 10 degree Celsius to 25 degree Celsius.
- The optimum temperature of C4 plant is 30-45 degree Celsius.
- C4 plants respond more to an increase in temperature as compared to C3 plants.
d) Water:
- Water stress causes stomata to close and hence, less carbon dioxide is available for photosynthesis.
- Water stress causes leaves to wilt, thereby reducing their surface area and metabolic activity as well.
II) Internal Factors
Internal factors of the plant which effect rate of photosynthesis are fallows.
- Amount or concentration of chlorophyll pigment in the leaf.
- Activity of enzymes for photosynthesis.
- The orientation of leaves, size of the leaf and number of leaves in a plant also affect the rate of photosynthesis.