THE EFFECT OF PHOTOINHIBITION ON PHOTOSYNTHETIC OXYGEN PRODUCTION IN THE BROWN ALGA DICTYOTA-DICHOTOMA

Photoinhibition of photosynthesis in the brown alga, Dictyota dichotoma, was studied with a PAM fluorometer (Walt, Effeltrich, Germany) and a homemade oxygen measuring device. As a measure of fluorescence, Fv/Fm, and for the photosynthetic yield, Delta F/Fm' were used. Oxygen measurements show clearly that the observed degree, as well as the time course, of photoinhibition depends on the fluence rate of the light used to measure changes of the production rate. After photoinhibition inhibition of photosynthesis the depression of oxygen production caused by non-saturating fluence rates was generally much more pronounced than that caused by saturating or nearly saturating fluence rates. At minimal photoinhibition the initial slope and the convexity of the fluence rate-response curve of oxygen evolution decrease, whereas the level of light saturation decreases only after strong photoinhibition. Nevertheless, at different degrees of photoinhibition, changes in the degree of the upper bending of the fluence rate-response curve of oxygen production are also linearly correlated to changes in the fluorescence ratios (Fv/Fm and Delta F/Fm'). The action spectrum of photoinhibition, calculated on the basis of changes of Fv/Fm, indicates that the reaction center of PS I is not involved in photoinhibition. The lower effectiveness of blue light in comparison to effects of green and red light may be due to chloroplast displacement, as in the so-called strong light position, the light absorbed by the thalli in vivo is decreased.