Bleaching of photosynthetic pigments in Chlorella protothecoides grown in the presence of SANDOZ 9785 (4-chloro-5-dimethylamino-2-phenyl-3 (2H) pyridazinone).

The unicellular green alga Chlorella protothecoides was grown in the presence of various concentrations (30–300 μΜ) of the herbicide SANDOZ 9785 (4-chloro-5-(dimethylamino)- 2-phenyl-3 (2H) pyridazinone). When the cells were grown autotrophically in the presence of the herbicide, there was a decrease in chlorophylls and carotenoids estimated on culture volume basis compared to the control. The absorption spectra of carotenes and colourless precursors extracted from photosynthetic particles showed that there was a higher carotenoid precursors/carotene ratio in the treated cells. Similarly the carotenoid precursors/xanthophyll ratio was increased by SANDOZ 9785 treatment. Chlorophyll a synthesis appeared to be inhibited more than chlorophyll b synthesis in SANDOZ 9785 treated cells resulting in a decrease in the chlorophyll a/b ratio.

Immediate effects of pyridazinone herbicides on photosynthetic electron transport in algal systems.

Pyridazinone herbicides, SANDOZ 9785 (4-chloro-5-dimethylamino2-phenyl- 3-(2H) pyridazinone), SANDOZ 9789 (4-chloro-5 (methylamino)-2-(α, α, α-trifluoro-m-tolyl- 3-(2H) pyridazinone) and SANDOZ 6706 (4-chloro-5-(methylamino)-2-(α, α, α-trifluoro-mtolyl- 3-(2H) pyridazinone) inhibited photosystem II electron transport in Chlorella protothecoides, when the herbicides were added to the assay medium. The inhibitory eficiency varied with the algal species and the nature of substitution of pyridazinones. Using 3 algal systems viz., Chlorella, Scenedesmus and Anacystis, the I50 value of for the inhibition of photosynthesis of 3 substituted pyridazinones (SANDOZ 9785, SANDOZ 6706 and SANDOZ 9789) were determined. SANDOZ 9789 was found to be the weakest inhibitor of photosystem II electron transport (H2Ο→ benzoquinone) as compared to SANDOZ 9785 and SANDOZ 6706. In general, the order of inhibition could be given as SANDOZ 6706 > SANDOZ 9785 > SANDOZ 9789. The I50 value of photosynthetic particles obtained from Chlorella cells was similar to that of whole cells, suggesting that the cell wall of Chlorella did not act as a barrier for the herbicide action. Studies on the light intensity dependence of SANDOZ 9785 inhibition of electron transport (H2O→ benzoquinone) showed that the light-dependent portion of the curve was more sensitive than the light independent portion of the curve. It is suggested that the site of action was on the reducing side of photosystem II.