Photoperiodism: An Effect of Darkness during the Light Period on Critical Night Length

The critical night length of the short-day plant Lemna perpusilla, grown with sucrose, increases roughly 3 hours under cycles with "light periods" composed of darkness preceded and ended by brief exposures to light. Although plants so grown are white, the effect is due neither to the absence of photosynthesis nor to insufficient total energy. It is inconsistent with current ideas on photoperiodic timing but may be explained by a hypothesis based on reported properties of phytochrome.