On the Mechanism of Phase Control by Light in the Rhythm of Carbon Dioxide Output in Leaves ofBryophyllum fedtschenkoi

Abstract

The induction of phase shifts in the rhythm of CO₂ output in leaves of Bryophyllum fedtschenkoi kept in continuous darkness and a CO₂-free air stream at 15 °C has been investigated by scanning the circadian cycle with 1-h and 3-h exposures to low fluence rates of red light. The experiments were designed to test the hypothesis (Wilkins, 1983) that phase-shift induction was achieved by the redistribution of malate between the vacuolar and cytoplasmic compartments of the leaf cells due to red light opening ‘gates’ in the tonoplast through which malate diffusion can take place. The use of red light exposures of two different durations enabled the direction of phase shifts to be established. From 8 h to about 22 h of darkness, when the cytoplasm would be expected to have a higher level of malate than the vacuole, only phase advances were observed, as predicted from the hypothesis. At later times in the cycle, phase delays and then phase advances were induced in a pattern closely similar to that reported for high temperature treatments (Wilkins, 1983). The results are discussed in relation to the tonoplast gate hypothesis which appears to account adequately for every feature of the phase shifts induced by exposing leaves to red light.