Temperature Sensitivity of Daily Respiratory Patterns Entrained through Phytochrome Action in Lemna paucicostata Strain 6746

The respiration rate of heterotrophic Lemna paucicostata Hegelm. 6746 cultures at 26–27°C and given a brief pulse of red light, assumes patterns with properties dependent on the nitrogen source supplied. In a search for conditions specifically affecting features of those patterns expressing photperiodic timing, their amplitudes have been measured as relative peak height (RPH) — the increased height at the daily peak as a proportion of the average daily minimum — at both 21°C and 28°C. On nitrate, ammonium or aspartate medium, RPH is reduced roughly 70% at 21°C as compared to 28°C, but on nitrogen‐deficient medium the reduction is only 20–30%. Yet growth and the actual intensity of respiration are not differently temperature sensitive on NO 3 and N‐deficient media. Nor does RPH itself correlate with growth rate. Previous evidence indicates that patterns on NO 3 and NH 4 media reflect photoperiodic timing and that those on aspartale and N‐deficient media do not; hence the temperature sensitivity of RPH does not correlate with whether or not a pattern reflects photo‐periodic timing. However, different daily patterns arc elicited by pulses of red or far‐red on NO 3 . NH 4 and aspartale media but not on the N‐deficient. Hence the temperature sensitivity of RPH does correlate with the degree to which the patterns distinguish between red and far‐red, which is to say between high anti low levels of Pfr‐phytochrome. This suggests that high temperature sensitivity in RPH reflects a reaction limited by N assimilation and saturating only at relatively high levels of Pfr.