A metabolic indicator of photoperiodic timing.

Characteristics of CO2 output patterns of axenic cultures of Lemna perpusilla are being sought that would serve as biochemically definable indicators of the crucial photoperiodic events during inductive light/dark schedules, using cycles with skeleton main light periods and overall periodicities of 24 hr. As the length of the main light period is increased over the range 4-10 hr, the daily times of maximal output on nitrate or ammonium media shift about 2.5 hr; there is a precisely parallel shift in the time of maximal photoperiodic sensitivity of flowering to a night interruption. These times do not depend on either a "drawn" or a "dusk" signal alone, but are functions of both. The time of maximal daily CO2 output on aspartate medium reflects only the dawn signal, suggesting that the display of some component of CO2 flux coupled to the photoperiodic timing mechanism can be controlled by modifying nitrogen metabolism.