Photoinhibition of photosynthesis in Lemna gibba as induced by the interaction between light and temperature. III. Chlorophyll fluorescence at 77 K

Photoinhibition in Lemna gibba L. was studied by interpreting chlorophyll fluorescence characteristics at 77 K on the basis of the bipartite model of Butler and co-workers (Butler 1978). Application of this analysis to chloroplasts (isolated from plants before and after exposure to a photosynthetic photon flux density of 1 750 μmol m^?2 s^?1 at 3°C for 2 h) revealed that photoinhibition had the following effect on primary events in photosynthesis. Firstly, the fluorescence of PS II increased (44%) in the state of open traps (F_o) and decreased (32%) in the state of closed traps (F_m). It is suggested, that the F_o-decrease reflects increased quenching by radiationless decay, both effects occurring at PS II reaction centers. Secondly, the rate constant for transfer of excitation energy from PS II to PS I (k_T(μ→J)) increased by 34%. However, in the state of closed traps, the flux of excitation energy via this transfer process decreased, most likely because of increased quenching by radiationless decay at PS II reaction centers. Thirdly, the probability for fluorescence from PS I decreased (19%). This indicates increased quenching by radiationless decay.