Open Access

Friederike Koenig

• Growth of the unicellular blue-green alga Anacystis nidulans in media containing sublethal concentrations of DCMU-type inhibitors of photosynthetic electron transport in strong white light gave rise to shade type appearance in this organism, as characterized by an increased ratio of phycocyanin to chlorophyll and reduced ratios, both, of carotenoids to chlorophyll and of total chlorophyll to P700. Shade type in Anacystis was caused neither by phenolic inhibitors tested nor by those known to bind to the cytochrome b6/f-complex. Surprisingly enough, the molar ratio of phycocyanin to chlorophyll in artificially shade adapted Anacystis1 grown in strong white light in the presence of 10-6 м atrazine, was found to increase with temperature for a given light intensity and with light intensity for a given temperature. Mutants of Anaeystis with a reduced binding capacity for DCMU-type herbicides due to an amino acid exchange in the 32 kDa Qв-binding polypeptide, also called D-1 protein, were ob- served to show shade type appearance in strong light, to respond very little to changes in light intensity and to show a reduced capability to further change their appearance to shade type by binding of competitors of Ob to the 32 kDa polypeptide. In Anaeystis a concentration of atrazine (10-7 м), ten times lower than the one causing the highest rate of shade adaptation (10-6 м), was shown to induce an optimum in cell density, which in turn resulted in an optimum in light-dependent O2 evolution. Both factors together might be responsible for the so-called greening effect observed in higher plants treated with sublethal concentrations of DCMU-type inhibitors of photosynthetic electron transport.