Biochem. Physiol. Pflanzen 187, 316-322 (1991) Gustav Fischer Verlag lena Diverse Effects of Two Organocarbamates Nematocides on Nitrogen Assimilation of Rhizobium japonicum 2002 in Free Living Culture D. K. MAHESHWARI and MEENU GUPTA Post-graduate Studies and Research in Microbiology, Department of Botany, Gurukul Kangri Vishwavidyalaya, Haridwar, India Key Term In d ex: Nematocides, nitrogen assimilation, respiratory inhibitors; Rhizobium japonicum Summary Results have shown that two nematocides carbaryl and carbofuran at all the concentrations were inhibitory to the growth of Rhizobium japonicum. An evaluation of bacteria after inoculating in media containing graded concentration of nematocides indicated a linear increase in percent mortality as the concentration of nematocides was raised. The rhizobia grown in nematocides supplemented medium exhibited increased rate of N0 3 uptake upto 30 min and maximum uptake was recorded in presence of 150 ml- 1 of carbaryl. The Lc50 of carbaryl and carbofuran were worked out to be 150 ml- 1 and 200 ml- 1 respectively in explanta studies. A range of concentration of both carbaryl and carbofuran showed stimulatory effects on nitrite reductase and nitrate reductase activity. Experiments conducted using energy inhibitors like DeeD (100 and HOQNO (100 alone and in combination with nematocides indicated a decline in the uptake as well as enzyme systems considered, which indicates the site of action of these organocarbamate nematocides at the respiratory level. Introduction Granular organocarbamate nematocides even at normal field rates affect adversely on certain macro (AGGARWAL et al. 1986; MAHESHW ARI and SINGH 1989) and microbial forms (ANDERSON, 1978). Variation of rhizobia to different pesticides and its macro symbiont have given varying results (RODELL et al. 1977). The nitrates are considered to be the primary source of nitrogen. Nitrates when transported into cell is converted to nitrite and subsequently to ammonia by nitrate and nitrite reductases, respectively (KOTHARI and SARAF 1987). The reduction of nitrate and nitrite consumes a large amount of energy and the electron donors are derived from oxidative energy metabolism (SINGH and SINGH 1984). It seems a difficult task to predict about the site of action of these organocarbamate nematocides (carbaryl and carbofuran) on the nitrate assimilation pathway; whether inhibitory effects of these carbamates is at the uptake Jevel (through membranes) or at enzymatic level. Respiratory inhibitors were also used to gain a further insight into the problem. Therefore, the present investigations were aimed at studying the effect of carbofuran and carbaryl on nitrate uptake and assimilation (nitrate and nitrite reductases) on Rhizobium japonicum 2002. Further, carbofuran and carbaryl are regarded as effective nematocides but their bactericide potential has not yet been studied on non-target organisms like Rhizobium species. Abbreviations: DeeD, N ,N' -dicyclohexylcarbodiimide; HOQNO,· 4(n-heptyl)-hydroxyquinoline- N-oxide) 316 BPP 187 (1991) 4