Biotoxic effects of the herbicides on growth, seed yield, and grain protein of greengram

Abstract

We studied the effects of atrazine, isoproturon, metribuzin and sulfosulfuron on plant vigour, nodulation, chlorophyll content, seed yield and protein content in seeds, in greengram inoculated with Bradyrhizobium sp. (vigna). The pre-emergence application of the four herbicides at 400 µg kg-1 of soil adversely affected the measured parameters. The average maximum increase of 10 % in seed yield occurred at 200 µg kg-1 of sulfosulfuron, while atrazine at 200 and 400 µg kg-1 of soil decreased the seed yield by 25 % and 40%, respectively. The average maximum chlorophyll content of 1.2 mg g-1 was obtained at 200 µg kg-1 of sulfosulfuron which declined consistently for all herbicides and increasing dose rates. Sulfosulfuron at 200 µg kg-1 increased the number of nodules found per plant by 7 % at 45 days after seeding the greengram. In contrast, the tested dose rates of atrazine, isoproturon and metribuzin significantly reduced the nodulation (nodule number and dry mass). The average maximum grain protein of 182 mg g-1 was obtained for sulfosulfuron at 400 µg kg-1 , while minimum grain protein was obtained at 400 µg kg-1-of isoproturon (124 mg g-1) and atrazine (125 mg g-1) application. Among the herbicides tested, atrazine and metribuzin showed a large degree of phytotoxicity to the crop, inhibiting its vegetative growth and was thus incompatible with greengram. @JASEM The nitrogen fixing bacterium Bradyrhizobium sp. (vigna) forms a specific symbiosis with greengram [Vigna radiata (L.) wilczek] and is commonly applied to the seed or soil as microbial inoculant to ensure functional symbiosis in this crop. Further, during the production of greengram, broad and narrow leaved weeds appear which adversely affect the productivity of greengram. The herbicides are therefore, required to control weeds in order to augment the yield. There are reports that suggest that herbicides when applied indiscriminately had variable effects on legume production (Khan et al., 2004). Moreover, the effects of herbicides are affected by the type and rates of its application, health and stages of plant growth, and other environmental variables. When applied frequently, the herbicides are accumulated in to the soil and at elevated levels impair the metabolic activities resulting in reduced growth of rhizobia, legumes or both. For instance, the photosynthesis inhibiting herbicide (metribuzin) affects the Rhizobium sp. (Heinonen-Tanski et al., 1982), the plant (Rennie and Dubetz, 1984) and the legume-Rhizobium symbiosis (Malik and Tesfai, 1985). Metribuzin and alachlor at recommended and five times more of recommended rates caused a significant reduction in nodulation, nitrogense activity and total N content in soybean (Malik and Tesfai, 1985). In a similar study, the recommended and double the rates of metribuzin inhibited growth, nodulation and N 2 fixation in cowpea without adversely affecting the N uptake by the plant (Silva et al., 1998). Fluchloralin and pendimethalin when applied under field conditions significantly decreased nodulation on soybean inoculated with Bradyrhizobium japonicum (Bollich et al., 1985).On the contrary, Billore et al., (2001) observed a significant increase in the dry matter accumulation in soybean following pendimethalin application in a two year field trial. In a similar study, fluchloralin when applied at normal rates, showed a considerable increase in nodulation on root system of groundnuts while the same rate of benefin, dinitroamine and nitralin decreased nodule mass, nitrogenase activity, yield and total N content, under field and greenhouse conditions (Durgesha, 1994). Geengram is an important grain legume crop of the world and is grown widely in the tropical countries. In India, greengram occupies an area of three million-hectare, accounting for 14% of total pulses area and 7% of total production (Singh et al., 2004). A relatively little information available on the effect of herbicides on greengram productivity formed the basis of this experiment. The present study evaluates the effects of commercial grades of atrazine, metribuzin, isoproturon and sulfosulfuron, on the Bradyrhizobium sp. (vigna) in pure culture and on vitality, nodulation, seed yield and grain protein of greengram, grown in clay pots. MATERIALS AND METHODS A total of 40 strains of Bradyrhizobium were isolated from nodules of greengram using standard methods (Holt et al., 1994). Strains of Bradyrhizobium were isolated using serial dilution plate technique on solid yeast extract mannitol (YEM) (g l-1 : mannitol 10; K 2 HPO 4 0.5; MgSO 4 .7H 2 O 0.2; NaCl 0.1; yeast extract 1.0; CaCO 3 2; pH 7) and were maintained on the same medium until use. The sensitivity/resistance of herbicides to Bradyrhizobial isolates was determined by plate dilution method. The herbicides used were: atrazine (gesaprim, e.c. 70%, Om Krishi,