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.

Alteration in the metabolism of free radicals in wheat seedlings grown in the presence of 4-chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone.

When wheat seedlings were grown in the presence of 62·5–500 μΜ 4 chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone, an inhibitor of photosystem II electron transport, there was a marked inhibition of in vivo photosystem II electron transport as revealed by the analysis of fast chlorophyll a fluorescence transients in intact leaves and by the inhibition (95% at 500 μM) of net photosynthesis in intact leaves Accompanying this inhibition of photosystem II electron transport, there was a decrease in the content of photosynthetic pigments. The extent of lipid peroxidation, measured in terms of malondialdehyde content was not increased; rather it was found decreased. An analysis of in vitro lipid peroxidation of the thylakoid membranes of control and 4-chloro-5-dimethylamino-2-phenyl-3(2H) pyridazinone treated plants in the presence of a sensitizer dye (toluidine blue) showed a similar rate both in the control and treated samples suggesting that the availability of unsaturated fatty acids as a substrate for lipid peroxidation was not limiting even though it decreased in the treated plants. On the other hand, it appears that the availability of the free radicals for lipid peroxidation was decreased byenhanced activity of the enzyme systems involved in the metabolism of free radicals. Measurements of the activities of enzymes involved in the metabolism of free radicals showed an increase in the activities of NADPH-glutathione reductase (6–8 fold) and catalase (15–30%) and a decrease in the activity of superoxide dismutase (30–45%) in the treated plants.

Increase in membrane permeability of electrolytes and betacyanin in beet root disc by fenitrothion.

Effect of fenitrothion (phosphorothioic acid, 0,0-dimethyl 0–4-nitro-m-tolyl ester), an organophosphorous insecticide, on membrane permeability employing the leakage of betacyanin and electrolytes as the criteria were studied in beet root (Beta vulgaris) discs. The leakage of both betacyanin and electrolytes increased with increasing concentrations (10-150 ppm) of fenitrothion in the incubation medium. At 0.33 mM the increase in electrolyte leakage was approximately linear for the first 6 h, while the increase in betacyanin leakage started with a lag of about 2 h. Long term incubation (24 h) showed a biphasic nature (in the semilog plot) for the increase in betacyanin leakage, while the increase in electrolyte leakage appeared more complex. In the control sample, the Arrhenius plots (25-50°C) of leakage showed a break at 40°C. In treated samples no breax was observed, but the slope decreased (for both electrolyte and betacyanin leakage) as compared to the respective slopes in the control in the temperature region greater than 40°C. The results are discussed in terms of the possible effect of the insecticide on the active transport in plant membranes

Correlation between inhibition of photosynthesis and growth of Chlorella treated with methyl parathion.

When methyl parathion (Ο,Ο dimethyl O p nitrophenyl phosphorothioate), an organophosphorous insecticide, was added to an exponentially growing culture of Chlorella protothecoides and the effects were followed for 12 days, the following observations were made: a) In autotrophic culture the cell number and the chlorophyll content decreased as compared to the control. These changes paralleled the inhibition of the rate of net photosynthesis, suggesting that the photosynthetic apparatus was the primary target of the insecticide action. b) The inhibition of cell growth (on cell number basis) also occurred in the case of heterotrophic culture at 100 μΜ insecticide concentration but the inhibition was less as compared to that of an autotrophic culture. c) The cell diameter in treated culture increased by 10 20% in both autotrophic and heterotrophic cultures. The observations, (b) and (c) suggest that apart from the photosynthetic apparatus, the insecticide has other sites of action, but the sensitivity of these sites to the insecticide is less than that of the photosynthetic apparatus.

The inhibition of photosynthetic electron transport by methyl parathion.

The effect of methyl parathion (metacid-50), an organophosphorous insecticide, on the Hill reactions of isolated mesophyll chloroplasts of Sorghum vulgare was studied. The pesticide was found to inhibit the Hill reaction with all the Hill oxidants tested, namely potassium ferricyanide,2,6-dichlorophenol indophenol and para-benzoquinone. The concentration of the pesticide required to inhibit 50% of the control Hill activity (I50value) was found to vary with the different Hill oxidants.