The toxicological effects of herbicide Pretilachlor on photosynthesis, respiration and nitrogen assimilation of the unicellular cyanobacterium Synechocystis sp. PUPCCC 64 has been studied. Treatment of the test organism with Pretilachlor (10, 15 and 20 mg L-1) negatively affected its growth, soluble proteins, photosynthetic pigments, photosynthesis and respiration in a dose dependent manner. Soluble cellular proteins decreased in range of 14-52%. Although herbicide affected all the photosynthetic pigments, maximum effect was observed on carotenoids (76% decrease) followed by allophycocyanin (61% decrease). Pretilachlor caused 49% decrease in photosynthetic rates. Studies on photochemical activity revealed that the herbicide affected both photosystems (PS-I and PS-II) as well as whole chain photosynthetic electron transport activity. The rate of respiration decreased in the range of 24-59% in the presence of herbicide. Decrease in photosynthetic as well as respiration rates ultimately resulted in decreased nitrogen assimilation as revealed by 50% reduction each in nitrate and nitrite uptake and 33% reduction in ammonium uptake along with 21-32% decrease in nitrate reductase, nitrite reductase and glutamine synthetase activity. The main toxic effect of Pretilachlor on the test organism appears to be on photosynthesis, and the effect on other physiological processes is a consequence of toxic effects of Pretilachlor on photosynthetic machinery.
Key words: Pretilachlor, Synechocystis sp., photosynthesis, respiration, nitrogen metabolism