i don't know if this is a stupid question but i can't seem to figure out the solution to the problem presented.

 

'You measure the photosynthesis speed of a North European plant. You measure it both under normal circumstances and at high concentrations of oxygen. Below are the results.

 

How is it that the net CO2 fixation is lower at a higher oxygen concentration?

 

I figure that it's because photosynthesis already produces O2 and if there's a high concentration of O2 already present then the plants won't take up CO2.

At the case of higher concentration of O2, the concentration of CO2 is lower and the respiration of the plant enhances.

I am not sure about it. Just a guess.

Plants need release the oxygen created during photosynthesis by diffusion, this will be slowed if the air has a higher oxygen percent.