One set of the plant was grown at 12 hours day and 12 hours night...

Short day plants (SDP) require a relatively short-day light period (usually $8-10$ hours) and a continuous dark period of about 14-16 hours for subsequent flowering. If these plants are exposed to day lengths in excess of this critical point, they continue growing vegetatively and do not show flowering. Most of the winter flowering plants belong to this category. For example, Rice, coffee, soybean, tobacco, sugarcane, strawberry and chrysanthemum. Related Theory In long that plants, it was found that they can flower even under short day conditions, if a flash of light is provided to the plant during the long dark period. Thus, periods of darkness have inhibitory effect on flowering in these plants.