RESUMO
Oscillatoria laetevirens produces an algicide, named oscillatorin (OS), which inhibits growth of higher plants. Effect of purified oscillatorin and some 'urea-triazine type' herbicides was studied on photosystem II activity and composition of pigment protein complex in spinach thylakoid membrane. For oscillatorin the I50 at 10 microg chlorophyll concentration, inhibitor constant (Ki), specific binding sites and Hill coefficient were calculated to be 1.45, 0.15, 2.3 and 0.2 microM respectively. Metribuzin and oscillatorin affected towards the donor side and brought about identical changes in polypeptide composition of PSII complex. Further, metribuzin and atrazine exerted antagonistic and synergistic responses on oscillatorin action. Some of these parameters were also studied on weed plants to assess upon the weedicidal potential of oscillatorin.