"Photosynthesis is the process whereby light is converted into chemical energy that the plant can use. Optimal photosynthesis for cannabis occurs at a 74-80 F (23-27 C). Any temperature below 65 or above 80 F (below 18 or above 29 C) is asking for trouble."
page 3, Marijuana Buds for Less: Grow 8 oz of bud for less than $100 by SeeMoreBuds