Does Photoinactivation of Photosystem II Occur in Low Light Conditions

Abstract

In studies of the mechanism of the photoinactivation of photosystem II (PSII), there are presently two conflicting hypotheses. One hypothesis is that excess energy received by leaves, neither utilized by photosynthesis nor safely dissipated in non-photochemical quenching, causes photoinactivation. The other involves a two-step mechanism in which excitation of Mn by photons is the primary cause of photoinactivation. In the former hypothesis, photoinactivation of PSII should not occur under low light with no excess energy conditions, but in the latter photoinactivation should occur. Therefore, we tested these two hypotheses under low light conditions. The leaves were photoinhibited in low light (30 μmol m−2 s−1) or in high light (950 μmol m−2 s−1) with blue, white, green or red LED lamps. The results showed that there was a difference in the extent of photoinactivation per photon exposure between low- and high-light conditions, and that photoinactivation did occur under low light with little excess energy condition. These results suggest that both mechanisms occur in the photoinactivation process.