I put 3 ulluco seedlings under 12 hour lighting to see what would happen. Each of them proceeded to produce a tiny tuber instead of true leaves:
We can add this to the list of reasons why sexual reproduction is rarely observed even in the Andes. Even if the plants do set seed, under short day conditions, those seedlings will probably not survive. In a pot in the greenhouse, I imagine that these tiny tubers will produce new plants after dormancy, but in the wild, such a small tuber at the surface of the soil would probably not survive to produce a new plant.
This may reveal something interesting about ulluco evolution as well. Did the plants originally come from a region with more variation in day length?