Development of acid tolerant strains of Bradyrhizobium sp. suitable for certain pulse crops of Kerala

Abstract

The present investigation on "Development of acid tolerant strains of Bradyrhizobium sp. suitable for certain pulse crops of Kerala" was conducted at College of Agriculture, Vellayani, Trivandrum during 1993-96. The initial isolation of acid tolerant strains of Bradyrhizobium suitable for cowpea, blackgram and green gram was done from seven different locations in Kerala. These locations were broadly grouped into category A and B. The root nodulation and plant dry weight were uniformly higher in category A locations compared to category B locations in all the three crops. In all, 43 native isolates of Bradyrhizobium were obtained including 17 from cowpea, 13 each from blackgram and greengram. The native isolates of Bradyrhizobium obtained were screened as per the Bureau of Indian Standard specification. The screening trial was done both under unamended and amended soil conditions. Thus in unamended soil, the isolates KA-G- C-4, VE-G-B-2 and PA-G-G-5 for cowpea, blackgram and green gram respectively were selected based on plant dry weight and in amended soil, the isolates KA-F-C-7, KA-F-B-6 and PI-G-G-8 for cowpea, blackgram and greengram respectively were selected for further studies.

The colony characteristics of the selected isolates were typical of Bradyrhizobium forming white translucent glistening and elevated colonies with entire margin on YEMA with congored. All •the six isolates were gram negative and fast growers attaining satisfactory growth on YEMA within three days. The carbohydrate utilization pattern showed that arabinose, a pentose sugar was the most preferred carbon source for the native isolates. At the sametime, the growth of - the exotic isolates were maximum in glucose. The ability to tolerate low levels of acidic pH upto 4.5 was more for the native isolates. The growth of both these isolates gradually improved and attained a peak level at pH 7.5. The in vitro tolerance to Fe and Al were almost identical for both the native and exotic isolates. In general, the growth was progressively inhibited with an increase in the concentration of these elements from 50 to 250 ppm. The antibiotic resistant markers for ampicillin, streptomycin and kanamycin were same for both the native and exotic isolates. The growth of these isolates were not inhibited at concentrations upto 1000 ppm of ampicillin, 250 ppm of streptomycin and 50 ppm of kanamycin. In the serological characterisation it was observed that one of the heterologous antigens (PI-G-G-8) for green gram showed positive agglutination with that of the cowpea isolate, KA-F-C-7. The effect of application of FYM, liming or pelleting on root nodulation and plant growth characters in cowpea, blackgram and greengram was studied. Significant increases in nodule number, nodule

dry weight and plant dry weight were obtained in the treatment combination of Bradyrhizobium inoculation along with FYM and lime application in all the three crops. An almost similar result was also obtained when liming was substituted with calcium carbonate pelleting. The efficacy of the selected isolates of cowpea, blackgram and green gram were tested under field conditions at two locations namely College of Agriculture, Vellayani and Rice Research Station, Kayamkulam during identical cropping season for two years. The seed treatment effects of the two selected Bradyrhizobium isolates for cowpea (KA-G-C-4 and KA-F-C-7), blackgram (VE-G-B-2 and KA-F-B-6), greengram (PA-G-G-5 and PI-G-G-8.) were evaluated with and without POP recommendations along with appropriate cont~ol treatments. In general, the root nodulation, plant growth and yield were significantly improved in all the three crops where Bradyrhizobium inoculation was practised along with the POP recommendation for each crop. Thus at Vellayani, the nodule number, nodule dry weight, leaf area, plant dry weight and yield in cowpea were maximum in the treatment combination of POP+KA-F-C-7. A similar response was also obtained with POP+KA- G-C-4 treatment. At Rice Research Station, Kayamkulam also, the yield increase was maximum in the treatment combinations of POP+KA- F-C-7 and POP+KA-G-C-4. In blackgram and greengram also the treatment effects were more or less similar to that of cowpea. In the last part of the present investigation, the influence of the POP recommendations on plant and soil nutrient status was studied.

The nitrogen content of plants was significantly higher in . Bradyrhizobium inoculated plants in cowpea and blackgram at both the locations. In greengram, at Vellayani there were no significant differences between treatments in the NPK content of plants. At Rice Research Station, Kayamkulam, also significant increases in nitrogen content of plants were obtained due to Bradyrhizobium inoculation. Unlike the nitrogen content a uniform treatment effect was not observed in the P and K content of plants. In the studies on soil nutrient status, it was observed that in general the availability of P, K, Ca and Mg were increased in the treatment combinations where POP recommendation was practised along with Bradyrhizobium inoculation. However, there was no significant difference between treatments in soil pH, organic carbon, Fe and Al content of different soil samples.