That looks like aluminum foil you're using to cover the walls and floor of your grow box. Get rid of it. Aluminum foil causes heat spots that will dry your plants to hell. I've had heat problems in the past, and although it has never gotten as bad as yours seems to have become, it will cause your leaves to droop badly. Without immediate intervention this heat stress will kill your plants, especially if you don't have proper ventilation. Use mylar instead of aluminum foil or even flat white paint. As mentioned above, over or under watering will also cause leaves to droop, but this to me appears to be more heat related. What are your temps at? It's important to keep your roots cool at all times. Also, fill up your pots to the rim with soil. This will help to insulate them from the heat.