[Effect of pyridazinone herbicides on the photosynthetic electron transport chain of chloroplasts and Chlorella].

In order to establish the site of pyridazinone herbicides action on the photosynthetic electron transport chain, their effect on the photochemical activity of chloroplasts and Chlorella was studied. It was shown that these compounds similar to diuron inhibit the delta F of chloroplasts but enhance the delta F and cause the disappearance of slow transient processes in Chlorella and change the light-off time course of delta F both in Chlorella and in the chloroplasts. The inhibiting effect is observed at herbicide concentration of 5 x 10(-6) M and is maximal at 10(-4) M. However, in contrast to diuron the herbicides enhance the msec afterglow in Chlorella cells; besides, even at concentration as high as 10(-4) M they only partly block photosynthetic oxygen evolution and the light-induced change of pH. Pyridazinone herbicides retard the delay of light-off delta F at-196 degrees C more efficiently than diuron. It is suggested that the herbicides under study inhibit the photosynthetic electron transport chain, however less efficiently than diuron; the inhibiting effect is decreased in the following order: SAN 9785, SAN 6706, SAN 9789. The herbicides affect mainly the acceptor part of the photosystem 2, retarding the electron transport from the intermediary acceptor to plastoquinone. In addition these herbicides may also have other sites of action in the region of photosystem 2.