It is the begining of heat stress or light stress The distance from light to plant looks good, I am not familiar with celcius so I can't say if it is to warm in there. I did see you recently put them under a HPS and by the looks of your grow you were using somthing with alot less lumins. So that leaves light burn and from what I see it is minimal and not a major concern yet. Put a fan in the room for air circulation and try using a NEW spray bottle ( not one that was used for any cleaner or chemical, residues from the previous chemical could have a devistating effect on your plants )and spray the plants occassionaly with just plain water to suppliment water to the leaves and help keep them cool.