EFFECTS OF INORGANIC NITROGEN ON THE RESPONSE OF LEMNA CARBON DIOXIDE OUTPUT TO LIGHT QUALITY AND TIMING *

—The effects of various light/dark schedules on the time course of CO 2 output by axenic cultures of the short‐day plant Lemna perpusilla 6746 differ substantially depending on whether the medium is N‐less or contains NH 4 or NO 3 as the sole N source. The steady‐state pattern achieved with a daily 1/4 h light pulse in N‐less medium is essentially the same whether the light is red or far‐red; on NO 3 or NH 4 , however, the red and far‐red patterns differ in form and suggest the action of a ‘Pfr‐hourglass’ timer. In darkness, following either continuous light or entrainment to kh red light daily, CO 2 output oscillates for three or more circadian cycles on NH 4 medium and for at least two on N‐less, but damps after a single cycle on NO 3 . A schedule of 1/4 h red light every 12 h elicits a 24 h periodicity on NO 3 or NH 4 media and a 12 h periodicity on N‐less medium, while a similar far‐red schedule elicits a 12 h periodicity on all three. CO 2 output patterns on each of the media respond differently to varying the daily span of light from 1/4 to 6 to 12 h. These results are probably due to differential effects of changing N status on the proportion of total C O 2 arising from various metabolic reactions. They suggest that, rather than being a simple, unitary indicator, CO 2 output can be made to reflect different processes on different media, increasing its value as a real‐time indicator of events underlying photoperiodism.