The effect depends on the plant and on its environment.
While more CO2 is good for plants, the associated changes in temperature and humidity can be bad and they can counterbalance the effect of increased CO2.
There has been a recently published study, for which I cannot remember a link, which has evaluated the current effects of the climate change for 7 major plant crops. For 3 of them, including rice, increased CO2 has resulted in higher productions, but for other 4, including maize, the associated changes in air temperature and humidity has determined lower productions, despite the higher availability of CO2.
While more CO2 is good for plants, the associated changes in temperature and humidity can be bad and they can counterbalance the effect of increased CO2.
There has been a recently published study, for which I cannot remember a link, which has evaluated the current effects of the climate change for 7 major plant crops. For 3 of them, including rice, increased CO2 has resulted in higher productions, but for other 4, including maize, the associated changes in air temperature and humidity has determined lower productions, despite the higher availability of CO2.