PhD

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
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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.