Studies of some strains of Rhizobium Sp. on some tropical legumes

Rhizobiun isolation were obtained from root nodules of Vigna unguiculata, Vigna radiate var aureus, Vigna subterranee, Fueraria sp. and Centrosema sp. and were designated as CP, GG, BB, PR and CT respectively. They were tested in the laboratory initially to comfirm their source and identity. Their cultural differences were established on yeast-water mannitel agar and other solid media. Also their susceptibilities to a set of antibioties were determined and used to differentiate between the isolates. The isolates were further tested for their levels of utilization of various carbohydrate substrates and the information obtained used to provide substitutes for mannitel – a common carbohydrate for rhizobia isolations and differentiation. The cross-inoculation concept has received much criticism in recent years. This was validated with the cross-infectivity and nodulation tests. This was done by inoculating a legume seedling with all the Rhizobium isolates from the other legume to look for nodulation Successful isolates on Vigna radiate var aureus and Vigna subterranea were further studied in the soil and their nodulation was observed to supply information on the following: (a) Pattern of nodulation; (b) Nodule surface characteristics; (c) Nodule sizes; (d) Nodule numbers ; and (e) Nodule dry weights. The nodules were further observed for their ability to fix nitrogen in the soil and the distribution of the fixed nitrogen to the plant part viz nodules, roots and shoots. In addition, nitrogen fixed in the soil was also evaluated to relate to the potentialities of the symbiotic system to either increase plant protein or soil nitrogen. The observations made were considered on the basis of the individual isolates and comparisons were made with each other and also with the untreated controls where applicable. The reactions of the isolates to the various laboratory tests, especially the cultural differences and susceptibilities to the set of antibiotics put the five isolates into two groups – isolates CT and CP in one group while isolates PR, BB, and GG were put to another group. Carbohydrate utilization confirms mannitel as the beat metabolized carbohydrate and also provided other useful substitutes such as inositol, galactose and glucose. An attempt to use differences in levels of carbohydrate utilization to separate the isolates, however, did not yield good results. The result of the cross-infectivity tests confirmed the claims by earlier workers to reject the concept of cross-inoculation groups, and especially the cowpea cross-inoculation group, as a mechanism of differentiating between Rhizobium strains. The concept could not hold true for the three botanical species. i. e. Vigna unguiculata, Vigna subterranea and Vigna radiate var aureus. Information supplied by the nodulation studies brought out the fact that host plants reacted differently to the isolates and this was clearly shown by the different nodule surface appearance and to some extent the number of nodules produced. On Vigna radiate var aureus, nodule surface characteristics produced by isolates PR and GG were closer to each other than they were to isolate CT which confirms the existence of subgroups within the group of the isolates used as already established in some laboratory tests. Differences between isolates on nodule number, size and dry weight were, however not significant at P = 0.05 thus maintaining the three isolates CT, PR, and GG in the same cross—inoculation group. Observed differences in the total nitrogen fixed by the different symbiotic system were not large enough to establish distinct specification among each set of isolates on the legumes used. These non-significant differences were also obtained for the inoculated and the uninoculated plants. From this, it could be assumed that the initial nitrogen content of the soil sample might have not permitted the expression of the full potentialities of the isolates in nitrogen fixation, thus providing and equally good growth medium for the uninoculated control plants.

Source: KNUST