31C leaf temp should still be okay but it's pushing it. There's some canna research out there that shows that the highest rate of photosynthesis occurs at 25-30 C, and beyond 30 it slows down substantially. So try to keep it at or below 30 if possible. This could be as simple as straightening out a duct line or something of that nature! Also, make sure your 4 or 6 hours lights off or whatever occurs at the hottest times of the day .... which is usually 1 pm - 5 pm, 12 pm - 6 pm.