lune777 this is probably referring more to the fact that Rubisco's affinity to bind with O2 and CO2 changes depending on the conditions, and in a sense, these two substates compete to bind to Rubisco's active site. Therefore, the two processes compete with each other. Plus, you're not just going to have one Rubisco enzyme doing all the work in the plant, but rather heaps, so at points, some may bind to O2 and some may bind to CO2. I guess some cells would be undergoing photosynthesis, whilst some may undergo photorespiration depending on the situation. It's all about relative affinity and the levels of substate present. Overall, there will be a swing towards one or the other, and in those hot/dry conditions, there are C4 and CAM plants to accommodate for its negative impacts on Rubisco (i.e. increased photorespiration). I hope that makes sense!
