Question

" The rate of photosynthesis increases with increase in intensity of sunlight till 0.02 % CO2 level and further increases with increase in intensity of sunlight only if CO2 level rises to 0.05%, " As per the video lesson and our book . But increase in CO2 level above 0.02% causes stomatal closure, which should reduce the rate of exchange of gases causing decrease in the rate of photosynthesis. Then why it's opposite happens?

Answer 1

Co2 concentration of the atm is 0.036% . Increase in its concentration up to 0.05% increases the rate of photosynthesis in most C3 plants.A decline is observed beyond 0.1%.when concentration is reduced there comes a point at which illuminated plants parts stop absorbing co2 from their environment. It is known as co2 compensation point or threshold valve.


stomatal control is linked not only to CO2 content of the leaf, but also regulates water loss through transpiration. This is a balancing act, whereby the stomata must admit sufficient CO2, whilst not allowing too much water to escape. In a higher CO2 atmosphere the plant is able to close the stomata somewhat and still achieve good internal CO2 levels, whilst reducing water loss, and it is the balance between these processes that matters, more than the actual ratio of internal to external CO2.