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Nitrate and the Course of Lemna perpusilla Carbon Dioxide Output under Daily Photoperiodic Cycles ¹

^a Biology Department, Brookhaven National Laboratory, Upton, New York 11973

The carbon dioxide output of Lemna perpusilla 6746 on modified Hutner's media under light (dark) cycles of 8 (16) hours is entrained in either of two forms depending on the level of nitrate. With nitrate low or absent, the peak occurs shortly after the start of each dark period, and the minimum, shortly after the start of each light period. With high (about 10 mM) nitrate, an additional or sole major peak occurs about 17 hours after the start of each light period; the minimum does not shift. This generalization probably also holds for cycles at least from 3 (21) to 18 (6). Apparent slow or unstable entrainment in some earlier data was undoubtedly a result of the progressive lowering of nitrate in the medium and, thus, of the nitrate peak. Future work under stable conditions, with or without the nitrate peak, should make possible the more accurate testing of models of entrainment and hypotheses concerning photoperiodic timing in this and related systems.