ORIGINAL PAPER Effect of Al and heavy metals on enzymes of nitrogen metabolism of fast and slow growing rhizobia under explanta conditions N. K. Arora • Ekta Khare • S. Singh • D. K. Maheshwari Received: 2 September 2009 / Accepted: 31 October 2009 / Published online: 20 November 2009 Ó Springer Science+Business Media B.V. 2009 Abstract Most of the legume crops are affected by metal stress present in the soil mainly due to contaminated agrochemicals and sewage sludge. The effect of alumin- ium, and heavy metals copper, iron and molybdenum on growth and activity of enzymes of fast and slow growing rhizobial sps. was studied. Sinorhizobium meliloti RMP 5 was found to be more tolerant to metal stress than Brady- rhizobium BMP 1 . Both the strains were extremely sensitive to Al than other metals. Al was much more deleterious for the enzymatic activities (nitrate reduction, nitrite reduction, nitrogenase and uptake hydrogenase) of strain RMP 5 and BMP 1 . Cu showed inhibitory effect on growth and enzyme activities of Bradyrhizobium strain at all concentrations. However, in S. meliloti RMP 5 all the tested enzymatic activities increased up to the concentration of 0.1 mM Cu. Fe enhanced the growth and enzyme activities of S. meliloti RMP 5 and Bradyrhizobium BMP 1 up to 100 mM concen- tration. Mo enhanced all the tested enzymatic activities of S. meliloti RMP 5 up to 1 mM. Nitrate and nitrite reduction activities of Bradyrhizobium BMP 1 increased up to 1 mM concentration. However, nitrogenase and hydrogenase activities of Bradyrhizobium BMP 1 got enhanced only up to 0.5 mM Mo. Both Fe and Mo are the key components of the enzyme nitrogenase and nitrate reductase and enhanced the growth and enzyme activities of both the sps. The study of physiology of nitrogen fixing ability of both fast and slow growing rhizobial strains reported that the supplementation of Mo and Fe in soils along with the biological formulations will enhance the process of sym- biotic nitrogen fixation. Keywords Hydrogenase Á Nitrate reduction Á Nitrite reduction Á Nitrogenase Á Nitrogen fixation Introduction In agricultural lands although biological nitrogen fixation is the sole alternative for chemicals, several environmental conditions are limiting factors for growth and activity of both the nitrogen fixing plants and root nodulating bacteria (Chen et al. 2003; Antoun and Pre ´vost 2005). In rhizobial cells there are two pathways for the assimilation of nitro- gen to ammonia. One pathway involves enzyme nitroge- nase, which directly converts atmospheric N 2 to NH 4 ? with reduction of protons and liberation of H 2 (Gupta and Maheshwari 1991). A H 2 -recycling mechanism utilizing uptake hydrogenase should increase the overall efficiency of N 2 -fixing process under conditions in which photosyn- thate is limiting (Schubert et al. 1977). The second pathway involves reduction of nitrate by the enzyme nitrate reduc- tase (NR). Subsequently nitrite is converted to NH 4 ? by the enzyme nitrite reductase (NiR) (Hervas et al. 1991). If nitrates are present in plentiful amount, legumes and their symbionts prefer the second pathway for the synthesis of amino acids (Sawhney et al. 1991). Optimum level of physiological function can be achieved in suitable environment and adequate nutrient supply (Wang et al. 2009). Some elements, such as heavy metals, though essential for organisms, are harmful if present in excess. Most of the cultivated legumes are exposed to agrochemi- cals, which not only contain essential nutrients but also N. K. Arora (&) Á E. Khare Á S. Singh Department of Microbiology, Institute of Biosciences and Biotechnology, C.S.J.M. University, Kanpur 208024, India e-mail: nkarora_net@rediffmail.com D. K. Maheshwari Department of Botany & Microbiology, Gurukul Kangri University, Hardwar 249404, India 123 World J Microbiol Biotechnol (2010) 26:811–816 DOI 10.1007/s11274-009-0237-6