Question

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

Answer 1

Rate of photosynthesis will increase because plant use carbon dioxide in photosynthesis

Answer 2

Increase in CO2 increases the rate of photosynthesis but only if not limited by other factors like light and temperature. If they are no limiting factors then the rate of photosynthesis increases. This increase is observed up to 1% of CO2 concentration. At the same time very high concentration of CO2 becomes toxic to plants and inhibits photosynthesis. So amount of CO2 released decides the rate of photosynthesis.