Answer:
1. 21 percent
2. Plants take in carbon dioxide from the air. But, since the amount of CO2 in the air is very less, it acts as a limiting factor for photosynthesis.
Explanation:
1. Increased photosynthesis results in more growth in some plants. Scientists have found that in response to elevated CO2 levels, above-ground plant growth increased an average of 21 percent, while below-ground growth increased 28 percent.
2. It is seen that, when light and temperature are not the limiting factors, increasing CO2 concentration leads to an increase in the rate of photosynthesis.