Carbon dioxide output as an index of circadian timing in Lemna photoperiodism.

Previous work on flowering suggested that photoperiodism in Lemna perpusilla 6746 involves an endogenous circadian "clock," but direct evidence requires study of an overt rhythm in the same plant. The CO(2) output rate of axenic cultures supplied with sucrose has been studied in a system using infrared analysis and monitoring four sets of cultures at once. Alternations of (1/4) to 21 hours of dim red light with darkness in 24-hour cycles can entrain the CO(2) output. In darkness following either continuous dim red light or entrainment to a 12(12) light (dark) schedule, the rate oscillates through two maxima and two minima, with a circadian periodicity, before apparently damping. In continuous red light, the rate is linear. The skeleton photoperiodic schedule (1/4)(5(1/2))(1/4)(18), with its two portions highly unequal, rapidly entrains the CO(2) output in a phase relationship which is the same irrespective of which dark period is given first. The schedules (1/4)(13)(1/4)(10(1/2)) and its inverse, however, with two portions more nearly equal in length, differ markedly from each other with respect to manner of entrainment, as they do in their effects on flowering. These and other results strongly support the concept that a circadian clock is an important component of photoperiodism, and they provide a new experimental system in which to study its action.