Phenotypic properties of some acid tolerant Faba bean (<i>Vicia faba L.</i>) rhizobia from central highlands of Ethiopia

Abstract

The study aimed to assess phenotypic properties of some selected acid-tolerant faba bean rhizobia from central highlands of Ethiopia. Thirty soil cores were taken from 0–20 cm and composited for soil analysis following standard procedures. The plant trap method was  used to isolate rhizobia. The purified colonies were used for eco-physiological, plant growth promotion (PGP) traits, and seed  germination test. The soils were strongly (4.8) to moderately acidic (6.25) with no risk of salinity. Organic matter was 2.6–4.69%, whilst  total nitrogen was 0.11–0.37% and the phosphorous availability was 12.88–63.97 cmolc/kg. The CEC was in the order of Ca>Mg>K>Na  where micronutrients were Fe>Mn>Zn>Cu and wider. Forty acid-tolerant isolates were obtained from five faba bean fields (collected from Fiche, Ginci, Gudar, Holota, and Midakegn). Of these, 15 isolates which showed 100% tolerance to pH 4 to 5 were selected for other  multiple PGP traits. Three rhizobial isolates (MFB5, FFB1, and MFB4) showed positive response for 8 PGP traits while FFB37 and GuFB8  which responded for 7 PGP traits were considered as the most potential in plant growth promoting properties. Rhizobial isolate FFB25  produced the highest indole-3-acetic acid (IAA) (174.63 ± 0.19 µg/ml) followed by MFB5 with 173.13 ± 0.14 µg/ml. Moreover, isolate FFB27  was known phosphate solubilizer (2 ± 0.025a) followed by MFB4 (1.7 ± 0.08ab) with the highest halo zone formation measured in mm.  Isolate obtained from Fiche (FFB37) was best in seed germination assay with 118.67% followed by FFB1 (86%) and FFB25 (75%). There was  a variation in % seed germination (72% and 56%) and vigor index (219.60 and 91.28) among inoculated and non-inoculated faba bean  seeds separately. This study suggests the potential of these rhizobial isolates for inoculum development following greenhouse and field  trials as soil acidity alleviation tools to boost supply of food and feed crops in the region.