The chlorophyll a (Chl a) fluorescence technique was applied to investigate damage of PS II during senescence of excised half-fronds in Spirodela polyrrhiza P143. The green explants showed a typical Chl a fluorescence transient, OJIP. After cultivation of explants under long-day conditions for 8 days, all the J, I, and P steps disappeared, but a clear K band, an indication of senescence, was observed. JIP-test showed that at this time point, the photosynthetic performance index (PI) dropped to zero and the active reaction center (RC) per leaf cross-section (RC/CS) declined to 18%. As the oxygen-evolving complex (OEC) and the chlorophyll content all remained above 42%, it is proposed that the decline in RC contributes more to the appearance of the K band. Supplementation of 6-benzyladenine (6-BA) into the medium at the beginning of cultivation caused dramatic increase in PI, OEC, RC/CS, and chlorophyll content, and at any time before the 8th day reversed the senescence process of the explants. When 6-BA was added after 8 days of cultivation, the PI did not increase anymore, RC/CS and OEC were maintained at 22% and above 40%, respectively, and chlorophyll content decreased continuously further. These data support a view that the decline in RC is crucial for initiation of the irreversible senescence phase of explants cultivated under long-day conditions. [ABSTRACT FROM AUTHOR]