Experiments with Salvia occidentalis, a short-day plant, showed that red nightbreak light which normally is effective in causing a long-day effect, can under certain conditions antagonize the long-day effect of a supplemental light period, thus causing a short-day effect. This short-day effect of nightbreak light disappears when the length of the nightbreak period is increased or when it is followed by an irradiation with far red (-= near infra red).