How does carbon dioxide concentration affect photosynthesis rate?

An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction - and so the rate of photosynthesis generally increases until limited by another factor.

Explanation:

CO2 concentration is directly proportional to rate of photosynthesis.

CO2 is raw material for process of photosynthesis.

CO2 + H2O → C6H12O6 + O2

Increase in CO2 concentration increases rate of photosynthesis.

Decrease in CO2 concentration decreases rate of photosynthesis.

Therefore, CO2 concentration is limiting factor for rate of photosynthesis.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.