MULTIPLE ACTION OF FAR-RED LIGHT IN PHOTOPERIODIC INDUCTION AND CIRCADIAN RHYTHMICITY

Abstract— It is generally accepted that phytochrome influences the photoperiodic induction of flowering through its interaction with the circadian clock mechanism. We have attempted to separate the effects of phytochrome on the clock mechanism from those that mediate flowering directly by examining a number of responses that are unrelated to flowering but are also regulated by the circadian clock. Gas exchange measurements of both CO2 and H20 vapor were monitored under light conditions (200 μmol m 2 s−1) where the addition of far‐red energy is required for the maximal promotion of flowering. In addition, photosynthetic capacity and maximal transpiration rates were measured in plants grown under continuous dim (20 μmol m−2 S−') light, with or without supplemental far‐red, by exposing them briefly to saturating fluxes (1000 μmol m−2 s‐l) of light. Net CO2 fixation was very weakly rhythmic in plants grown under both high and low light and this weak oscillation was completely suppressed by far‐red light. Far‐red also suppressed the rhythm in transpiration under high light, but the rhythm was immediately reinstated when the far‐red light was removed. The phase of this rhythm was also reset with the next peak always occurring15–18 h after the far‐red was turned off. When grown under dim light, the transpiration rhythm was not suppressed and the amplitude of the oscillation was more than doubled. Far‐red light appears to interact with the rhythm in transpiration in a manner suggesting that the stomatal rhythm may be coupled to the same clock oscillator that regulates the flowering rhythm. Copyright © 1990, Wiley Blackwell. All rights reserved