Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa.
Electron flow around photosystem II was investigated in Chlorella pyrenoidosa. Using a bare platinum O(2) electrode, simultaneous measuremnts were made of steady-state photosynthesis in continuous light, the yield of oxygen (Y(o(2) )) produced by a superimposed saturating xenon flash, and the change in fluorescence yield of a weak flash triggered before and 70 microseconds after the saturating flash. Throughout most of the continuous photosynthesis-irradiance curve, normalized O(2) flash yields (Y(o(2) )/Y(o(2)max)) and normalized variable fluorescence yields (Deltaphi/Deltaphi') were linearly correlated with a slope of 1.0. As photosynthetic rates reached light saturation, however, the variable fluorescence yields remained relatively constant while O(2) flash yields decreased. These results strongly suggest that there is a cyclic electron flow around photosystem II in unpoisoned intact cells at light saturation and supraoptimal light intensities.
Falkowski
P G
PG
Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973.
Fujita
Y
Y
Ley
A
A
Mauzerall
D
D