Photosynthetic Properties of Green Barley Leaves after Treatment with Pyridazinone Herbicides—Comparison with the Effects of Diuron

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Photosynthetic characteristics of detached green barley leaves after 72 h of treatment with 0·2 mol m−3 of the pyridazinone herbicides SAN 6706, SAN 9785 and SAN 9789 were studied. For comparison, the effects of 0·01 mol m−3 diuron were also investigated. Pyridazinone herbicides caused only a slight reduction of the total carotene content of the barley leaves. The total chlorophyll content, as well as the linolenic to linoleic acid ratio of chloroplast glycerolipids, however, remained unchanged. Diuron treatment caused total inhibition of electron transport, as revealed by fast fluorescence induction of leaves and the Hill reaction activity of chloroplasts. The 14CO2-nxation by the leaves and the light-induced fluorescence quenching were also completely inhibited in vivo by diuron. Pyridazinone herbicides left 20–40% of the 14CO2-fixation found in the control, in spite of the fact that their fast fluorescence induction tracings showed inhibition in the electron transport. Chloroplasts isolated from the leaves treated with pyridazinones were found to be highly active in mediating the ferricyanide-dependent Hill reaction. In order to test the ability of pyridazinones to inhibit photosynthetic electron transport in vivo, their ‘prompt’ effect on fluorescence was also investigated. It is concluded that pyridazinone herbicides can readily and rapidly enter the chloroplasts and inhibit the photosynthetic electron transport in vivo. The differences between the long-term effects of pyridazinones and those of diuron suggest differences in the inhibitory effectiveness on the various photosynthetic parameters between the two herbicide groups. It is suggested that pyridazinones can leave the chloroplasts during isolation owing to the loose binding onto the thylakoid membranes.