Question

When the ratio of O2 to CO2 within their leaves _______, C3 plants undergo a shift toward more _______ and less _______.

Answer 1

When the ratio of O2 to CO2 within their leaves INCREASES, C3 plants undergo a shift toward more PHOTORESPIRATION and less CALVIN CYCLE.
In C3 plants increase in the concentration of O2 leads to photorespiration. Increase in the amount of CO2 decreases decreases the rate of photorespiration.