Résumé
Cyclic electron flow around photosystem (PS) I has been
widely described in vitro in chloroplasts or thylakoids isolated from
C3 plant leaves, but its occurrence in vivo is still a
matter of debate. Photoacoustic spectroscopy and kinetic
spectrophotometry were used to analyze cyclic PS I activity in tobacco
(Nicotiana tabacum cv Petit Havana) leaf discs
illuminated with far-red light. Only a very weak activity was measured
in air with both techniques. When leaf discs were placed in
anaerobiosis, a high and rapid cyclic PS I activity was measured. The
maximal energy storage in far-red light increased to 30% to 50%, and
the half-time of the P700 re-reduction in the dark
decreased to around 400 ms; these values are comparable with those
measured in cyanobacteria and C4 plant leaves in
aerobiosis. The stimulatory effect of anaerobiosis was mimicked by
infiltrating leaves with inhibitors of mitochondrial respiration or of
the chlororespiratory oxidase, therefore, showing that changes in the
redox state of intersystem electron carriers tightly control the rate
of PS I-driven cyclic electron flow in vivo. Measurements of energy
storage at different modulation frequencies of far-red light showed
that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco
mutant deficient in the plastid Ndh complex was kinetically different
from that of the wild type, the cycle being slower in the former
leaves. We conclude that the Ndh complex is required for rapid electron
cycling around PS I.