Functional changes of photosystem II in the moss Rhizomnium punctatum (Hedw.) induced by different rates of dark desiccation

An effect of different rates of dark desiccation on Photosystem II (PSII) in the drought medium-tolerant moss Rhizomnium punctatum (Hedw.) was studied by exposing the detached leaves for 24 h to air of relative humidities of 85%, 70% and 30%. A similar decrease in O-2 evolution after 24 h of desiccation, but diverse changes of the O-J-I-P chlorophyll fluorescence transient during desiccation, were observed. Whereas the Fp value remained fairly constant, the Fo value increased with increasing time and rate of desiccation, leading to a decrease in che Fv/Fp value for 70% and 30% RH. Concerning the relative height of the J step, two different trends in the dme courses of the rFj parameter were found. At 85% RH, only a slight rFj increase was observed while at 30% RH an rFj decrease dominated. The samples exposed to 70% RH exhibited both phases: the increasing one was followed by the decreasing one. The Western-blot method indicated that no D1 degradation occurred during desiccation. Similarly, no changes in the E475/E436 band ratio of the Chi fluorescence excitation spectrum at 77K were found for 30% RH. It is suggested that damage to the acceptor side of PSII dominating at 85% RH is followed by a functional disconnection of P680 from the antennae systems, which was apparent at higher rates of desiccation. The notion of different reactions to fast and slow desiccation was further supported by the experiment with the chemically closed PSII.