When light intensity and other factors are favourable, the rate of photosynthesis is directly proportional to the duration of light. It has been found out that the quantum yield is higher when plants are exposed to continuous daylight of 10 – 12 hours. Finding of Mitchell (1936), Boehring (1949) and others have indicated that plants continue photosynthesising for long periods without any ill effect on them. Devlin and Baker (1971) however have suggested that continuous light is damaging to photosynthesis.
The rate of photosynthesis has been noted to be high under diffused light for longer duration, than under high intensity of light for shorter duration.