The diurnal changes of chlorophyll fluorescence of terrestrial plants have been well studied and show a midday depression in summer. However, only a few studies have been performed on submerged macrophytes though they play important roles in shallow lakes. In the present study, six common submerged macrophytes were chosen to investigate the diurnal changes of the maximum potential quantum efficiency of photosystem II (Fv/ Fm) on sunny days in summer. The Fv/Fm values of all these species decreased at midday under natural conditions, but there was no significant change either under the “Shade” treatment or under laboratory conditions.
The Fv/Fm values were negatively correlated with photon radiance, except for Ceratophyllum demersum, which probably could support high light as shown by rapid light curves. The decreasing Fv/Fm values during midday known in terrestrial plants to be caused by either down-regulation or photodamage of photosystem II as a response to high irradiances was here also demonstrated for submerged freshwater macrophytes. In Vallisneria denserrulata showed an additional component of circadian rhythm to the midday photoinhibition. Circadian periodicity and the diurnal changes of the photon irradiance should be considered when investigating the
chlorophyll fluorescence of submerged macrophytes.