This isn't necessarily an immediate problem for plants, because plants store extra calories generated from photosynthesis for future use, and when the temperature is too hot they can switch from photosynthesis to respiration, consuming their stored sugars for energy. During this time, plants become net producers of CO2 instead of net consumers, taking in oxygen, burning their sugars (in the same slow chemical burn we use) and releasing the CO2 at the end.
This is an immediate concern for us, because those stored calories are what we wanted to eat. Plants may survive higher temperatures, but they produce fewer stored calories in the form of delicious fruits, vegetables and grains for us to eat. Climate change is complicated for agriculture, but extra days above (or close to) 40C is just bad, bad, bad.