INOCULATION WITH RHIZOBIUM STRAINS IMPROVE COMMON BEAN PHASEOLUS VULGARIS TOLERANCE OF HYDROUS CONSTRAINT

In many African countries, drought is a major environmental stress affecting agricultural productivity. The potential solution to this problem for the common bean is to enhance its yield by the effective use of rhizobia. To improve the osmotic stress tolerance of a drought-sensitive common bean cultivar (Coco Blanc) consumed in Tunisia, plants were inoculated either with the reference strain Rhizobium tropici CIAT899 or with the local soil Rhizobium strain Ar02. Plants were grown under well-watered or medium to severe water-deficit conditions (100%, 75%, 50%, and 25% of the pot's useful reserves). Nodulation parameters measured included nodule biomass; growth (length and biomass of shoots and roots), leaf area, leaf perimeter, nitrogen content, chlorophyll and carotenoid content, soluble sugar content, and proline content were measured at the flowering stage. A significant reduction in parameters was observed due to water shortfall. However, plants inoculated with Ar02 and CIAT899 showed some tolerance to water stress; the number of nodules increased from 1 nodule/plant at 50% Useful Reserve (RU) to 28 nodules/plant at 50% RU + CIAT899. At 50% RU, the reduction in nodule biomass was 92.57% in plants inoculated with Ar02 and CIAT899 and 99.85% in control plants. Inoculation with CIAT899 increased dry biomass at 75% RU, 50% RU, and 25% RU and root length at 75% RU and 50% RU compared to the control treatment. Inoculation with CIAT899 also increased leaf area and perimeter. The effect of drought stress was lower in plants inoculated with rhizobia as demonstrated by chlorophyll a, chlorophyll b, chlorophyll a + b and carotenoid content. Rhizobia inoculation, especially with Ar02 at 50% RU and CIAT899 at 75% RU, increased nitrogen content. Inoculation also increased the soluble sugar and proline content. This study suggests that rhizobial inoculation of the common bean improves its performance and stress tolerance under adverse conditions.