Photoperiod

One of the bits of conventional wisdom that floats around aquarium articles and message boards is that tropical plants, which are adapted to a 10-14-hour photoperiod, and that anything beyond that is a bad idea. This article is a typical example:

Leaving the lights on 24 hours a day while you were away was not a good idea. Most of the plants we use in the aquarium come from tropical areas, and are adapted to about 10 and 14 hours each of day and night. At 2 to 3 watts per gallon of fluorescent light, a 12-hour photoperiod works well for me. This can be adjusted based on lighting intensity. In no case does it make sense to extend the photoperiod much beyond 14 hours. Most higher plants will stop photosynthesizing at this point, while the algae will take full advantage of this situation.

But is this really the case? Does photosynthesis shut down after 12 hours? Possible, but what's the source? I need to figure this out.