Abstract
The biodiversity of rhizobium in soils of the São Francisco Valley is unknown and can be studied using cowpea as trap plants. The objective of this study was to verify the diversity of diazotrophic bacteria that nodulate cowpea in soils of the lower half of the São Francisco River Valley by morphological and genotypic characterization. Seven soil samples (A1, A2, A3, A4, C1, C2 and MC) were collected to capture bacteria associated to five cowpea cultivars (IPA 206, BRS Pujante, BRS Marataoã, Canapu Roxo, and Sempre Verde), in a 5x7 factorial design with three replications. Thirty days after plant emergence, the nodules were collected and the bacteria isolated and analyzed in relation to their growth characteristics in YMA medium. The 581 isolates were grouped in 49 morphologic groups. Of this total, 62.3 % formed colonies in up to three days, 33.4 % grew from the 6th day on, and 4.3 % began to grow 4 to 5 days after incubation. Regarding the formation of acids and alkalis, 63 % acidified the medium, 12 % made it alkaline and 25 % maintained the medium at neutral pH. The highest diversity was observed in the A3 sample and in isolates associated with the cultivars Canapu Roxo and BRS Pujante. Thirty-eight representative isolates were chosen for the genotypic characterization, clustered in four groups based on the restriction analysis of 16s rDNA. This grouping was strongly correlated with the sampling site; 13 rhizobium isolates had an electrophoretic profile distinct from the standard rhizobium strains used in this study.
Highlights
The symbiosis between leguminous plants such as cowpea [Vigna unguiculata (L.) Walp], and N2 fixing bacteria that form nodules (Rhizobium group) in the soil, is a biological phenomenon that needs to be maximized in agricultural systems
The rhizobial population that nodulates Phaseolus vulgaris was studied by Anyango et al (1995), in two soils in Kenya (Africa), and this population seemed to be similar in number and N2 fixation effectiveness, but the composition differed
BRS Pujante seemed promising for biological N2 fixation (BNF)-related studies, for promoting a significant gain in shoot dry matter induced by seven isolates when compared with N
Summary
The symbiosis between leguminous plants such as cowpea [Vigna unguiculata (L.) Walp], and N2 fixing bacteria that form nodules (Rhizobium group) in the soil, is a biological phenomenon that needs to be maximized in agricultural systems. The diversity of rhizobium species, along with the variable edaphoclimatic conditions and the genetic specificities of cowpea varieties, among other factors, make these bacteria more or less effective for biological N2 fixation (BNF). The rhizobial population that nodulates Phaseolus vulgaris was studied by Anyango et al (1995), in two soils in Kenya (Africa), and this population seemed to be similar in number and N2 fixation effectiveness, but the composition differed. According to Martins et al (2003), the rhizobial population in the soil can be extremely variable, both in the composition and the symbiotic characteristics of a species. Nodules formed by different strains and even by different species may occur in the same plant (Moreira & Siqueira, 2006)
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