An insight into cadmium's effect on the process of photosynthesis was obtained by determining the time course of cadmium effect on different generations of the great duckweed (S. polyrhiza L. Schleiden). Cadmium has caused disturbances of the thylakoid structure and senescence of older fronds. It caused structural irregularity and an increase in starch grain size in younger fronds, even at the lowest concentrations. The hormetic effect of lower Cd concentrations on respiration and photosynthesis rate was due to the distribution of available energy by activating mechanisms involved in mitigating Cd's adverse effects. The diversion of substrates to storage and an increase in respiration rate resulted in a reduced growth rate. The index of total photosynthetic efficiency (PITOT) proved to be the most informative photosynthetic efficiency parameter, indicating that Cd affects effective energy conservation in older and younger plants. A more detailed insight into the influence of Cd on electron transfer was provided by the values of JIP test parameters related to the overall PSII function (VK and ΦP0), the PSII acceptor side (VJ, ΨE0 and ΦE0), and the overall operation of PSI (VI, ΨR0 and ΦR0), with PSI, showed to be more influenced by Cd. Younger frond's potential to sustain photochemical events, including the activity of oxygen-evolving complex and electron transport processes, at higher cadmium concentrations than at older maternal fronds enabled young plants to cope more effectively with cadmiuminduced stress. This study's results may contribute to a more precise and representative determination of cadmium's toxicological profile and the characterization of plant responses to stress conditions.