There is actually a lot going on physiologically with "Circadian Rhythms in Plants" below are some quotes taken from an article that anyone considering Non-Standard light cycles should read.
"Plant scientists have shown that a properly tuned circadian clock is important for plant disease resistance to arrays of pathogens and pests.....
Even in the absence of pathogens, many of these responses show low but rhythmic changes that are influenced by the circadian clock. When a real attack arrives, the plants’ daily rehearsal of their defense systems ensures a strong and concerted timely defense. Plants with misaligned clocks succumb to the attack."
On another slightly unrelated note. Tropical landrace strains grow in areas that don't get more than 13 hours of sunlight or more than 11 hours of darkness.