TEMPERATURE DIFFERENTIAL. “In most of the species studied, petal movements are due to a difference in growth rate of the two sides. In species that open in the morning and mainly respond to temperature, for example, the inner surface of the petal rapidly grows in length when the temperature is raised, whereas the outer surface does not. Cooling results in a more rapid growth of the outer surface.”
http://jxb.oxfordjournals.org/content/54/389/1801.full