like :
1. co2 concentration in atmosphere
2. water available to the plant
3. no. of leaves present on the plant ( i.e. no. of chloroplast )
4. intensity of light
5. region where the plant is grown
6. temperature etc.
hence, it is not possible to find an average amount of co2 accepted and o2 liberated by plants as it is also dependent on no. of leaves present on plant.
A tree can absorb about 50lbs of CO2 per year