The rate of photosynthesis increases linearly with an increase in the intensity of light.
However, extremely high intensities of light do not increase the rate of photosynthesis. Optimum light intensity for photosynthesis varies with the species of the plant. In fact, very high light intensity may bleach chlorophyll and retard photosynthesis.
Carbon Dioxide Concentration:
In normal conditions, carbon dioxide is the major limiting factor in photosynthesis. The rate of photosynthesis increases with an increase in the C02 concentration. The concentration of C02 in the atmosphere varies from 0.03-0.04 per cent. A concentration of 0.02 per cent of C02 for short duration is optimum for increasing the rate of photosynthesis.
However, over long periods even 0.05 per cent C02 concentration in the atmosphere can increase the rate of photosynthesis provided the light intensity is also increased to support it.
In general, increase in temperature results in an increase in the rate of photosynthesis when other factors are not limiting. Photosynthesis is restricted to a temperature range in which the enzymes remain active. Further a rise of 10 °C up to optimum temperature (35 °C) doubles the rate of photosynthesis, for e.g., a rise from 25 °C to 35 °C doubles the rate of photosynthesis.
The maximum suitable temperature when photosynthesis occurs best is about 35 °C above which the rate falls. The process of photosynthesis falls and stops above 40 °C as the enzymes get destroyed. Similarly, low temperature also inhibits enzymatic activity and rate of photosynthesis is reduced.
Less than 1 per cent of the total water absorbed by plants is utilized as a raw material in photosynthesis. Water rarely becomes a limiting factor in photosynthesis.
2. Internal Factors:
Inadequate amount of nutrients like minerals causes loss of chlorophyll in leaves thereby reducing the trapping of solar energy.
ii. Structure of leaf:
Size and thickness of leaf and distribution of stomata influences the amount of C02 and light entering the leaf.
Dehydration of protoplasm and accumulation of sugar and starch in the leaves reduces the rate of photosynthesis.