Assessing the Effectiveness of Native Rhizobia as Potential Strains for Local Inoculant Production for Enhanced Cowpea and Groundnut Yields in Northern Ghana
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Date
October, 2018
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Abstract
Efficient exploration of the legume-rhizobium symbiosis via inoculation with rhizobia is
constrained by the limited knowledge on the fate of introduced strains under field
conditions and the unavailability of effective native strains for use as local inoculants.
This research therefore sought to address these challenges in four studies: i) nodule
occupancy determination ii) identification of effective native isolates iii) symbiotic
performance evaluation of elite native isolates under field conditions and iv) assessment
of the persistence of elite isolates following field inoculation.
Significant differences (p< 0.05) in shoot biomass of cowpea following inoculation was
observed. The differences were explained via nodule occupancy studies by designing
specific primers for the test strain Bradyrhizobium pachyrhizi BR 3262 (a recommended
cowpea strain in Brazil) using the comparative genomics approach. Out of eleven specific
primer pairs designed, the primers 2645 and 2736 were observed to be highly sensitive
and reliably detected the target strains in nodules extracts of gnotobiotic system and potted
soil grown cowpea. These two primers are thus novel tools for determining the fate of BR
3262 in field inoculation studies. The approach for designing specific primers in this study
should be applicable to other bacteria/rhizobium strains whose genomic sequences are available.
Bioprospecting for nodules of groundnut and cowpea cultivated in farmers’ fields without
inoculation was done followed by isolation of rhizobium in the laboratory. Authentication
and symbiotic effectiveness evaluation of the isolated rhizobium strains led to the
selection of seven effective isolates. The nitrogen accumulated via BNF of the selected
effective isolates was significantly higher (p< 0.05) compared to the –N control treatment.
Treatment with isolate KNUST 1002 resulted in a total N accumulation that was comparable to the reference strain 32H1 on groundnut (BR 1 variety) grown in potted
soils. Genetic characterization of the seven effective isolates revealed them as diverse with
isolates KNUST 1003 and KNUST 1007 belonging to the Rhizobium tropici speices. The
five remaining isolates were identified to belong to the Bradyrhizobium yuanmingense
species.
The two most effective isolates were evaluated in the field alongside a positive (with
nitrogen) and negative (without nitrogen or inoculation) control on cowpea (Songotra variety) and groundnut (Chinese variety) in multi-locational trials in the Northern region
of Ghana using the randomized complete block design. On the average, cowpea grain
yields produced by the inoculated treatments were significantly (p< 0.05) larger than the
–N treatment. Isolate KNUST 1002 produced the best average yield (1056 kg ha-1
) which
was significantly different from the other treatments. Groundnut yields on the other hand
were significantly larger with isolate KNUST 1006 (1234 kg ha-1
). The chemical
properties of soils in the study locations generally showed low soil fertility. The different
soil types significantly influenced the performance of treatments across the study
locations. Twenty-six and 23% of the variability in grain yields of cowpea and groundnut
respectively, was explained by the soil type and treatment interaction. These results imply
that effective native isolates in this study can improve grain yields of cowpea and
groundnut based on location specific recommendations.
The persistence of effective isolates following previous inoculation on cowpea and
groundnut was assessed in the 2017 cropping season on ten of the sites considered in 2016.
Nodulation in both previously inoculated and re-inoculated plots were significantly higher than the un-inoculated plots (+N and –N). Nodule dry mass recorded for isolate KNUST
1006 in previously inoculated plots did not differ significantly from that of re-inoculated
plots on both target host. However, the nodule dry mass recorded for isolate KNUST 1002
on cowpea in previously inoculated plots was significantly lower than in re-inoculated
plots. Increases in grain yield of groundnut were significantly larger for treatment with
isolate KNUST 1006 than the other treatments for previously inoculated and re-inoculated
plots. Isolate KNUST 1002 on cowpea produced significantly higher yields than the other
treatments in previously inoculated and re-inoculated plots. The type of legume host in
each case influenced the performance of the isolates. There were no significant differences
in the yields produced by either of the isolates in previously inoculated and re-inoculated
plots. These results indicate that effective native isolates persisted and effectively nodulated the target legumes obviating the need for re-inoculation in subsequent cropping
season.
The outcomes of these studies have important implications for the use of effective isolates
from this study as inoculants to improve cowpea and groundnut yields particularly in Northern Ghana.
Description
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in the partial fulfillment of the requirements for the award degree of Doctor of Philosophy in Soil Science.