Question

if we keep on increasing CO2 concentration in air what will be the rate of photosynthesis

Answer 1

it will be low.......

Answer 2

This growth stimulation occurs because CO2 is one of the two raw materials (the other being water) that are required for photosynthesis.  Hence, CO2 is actually the "food" that sustains essentially all plants on the face of the earth, as well as those in the sea.  And the more CO2 they "eat" (absorb from the air or water), the bigger and better they grow.