Low light acclimated submerged freshwater plants show a pronounced sensitivity to increasing irradiances

The high light sensitivity of three submerged aquatic freshwater plant species, Egeria densa, Elodea nuttallii and Myriophyllum heterophyllum, which have been cultivated at a photosynthetically active radiation (PAR, 400–700 nm) of 70 μmol photons m−2 s−1, was studied by means of chlorophyll fluorescence and pigment analyses. Exposure of plants to 100, 300, 600 and 1000 μmol photons m−2 s−1 PAR for up to 360 min induced a strong reduction of the Fv/Fm ratio, indicating a pronounced inactivation of PSII even at the lowest PAR applied. These changes were accompanied by a reduction of the chlorophyll content to about 60–70% of control values at the highest PAR. Rapidly inducible photoprotective mechanisms were not affected, as derived from the rapid generation of pH-dependent energy dissipation under these conditions. At PAR higher than 100 μmol photons m−2 s−1, however, the primary quinone acceptor of photosystem II, QA, was reduced to about 80% and the effective quantum yield of photosystem II, ΦPSII, dropped to values of about 10%, indicating a high reduction state of the photosynthetic electron transport chain. These data support the notion that the three aquatic macrophytes have a very low capacity for the acclimation to higher light intensities.