At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at similar to 0.2-1 and M at similar to 100-500 s, with a minimum S at similar to 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with 0 denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS -> Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS -> PS I excitation transfer. Blocking the PBS -> PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K. Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobactrial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P T fluorescence decay, reminiscent of the typical P -> T fluorescence decay of higher plants and algae. A similar P -> T decay was also displayed by DCMU-treated Synechococcus cells at 2 degrees C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2 -> 1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state I and upon illumination they display an extended P -> T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1 -> 2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS -> PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Svnechococcus becomes plant-like. (c) 2007 Elsevier B.V All rights reserved.
