Module 3: Demonstration 2

MODULE 3: DEMONSTRATION 2

GROWTH CHARACTERISTICS OF RHIZOBIA

PURPOSE

 To demonstrate that rhizobia can be broadly classified into two main types based on growth rate and growth reaction on yeast mannitol agar (YMA) medium containing indicators (bromthymol blue and congo red).  To demonstrate the growth and appearance of rhizobia on YMA from different cross-inoculation groups.  To demonstrate the growth and appearance of rhizobia cultured in liquid yeast-mannitol (YM) medium.

CONCEPTS FROM DEMONSTRATION

The two main classes of rhizobia are:  fast-growing rhizobia with an acid reaction (bromthymol blue indicator turns yellow)  slow-growing rhizobia with an alkaline reaction (bromthymol blue indicator turns blue)

Rhizobia show little or no congo red adsorption and appear faintly pink to white. Nonrhizobia absorb congo red and appear as red colonies.

Rhizobia that are fast-growing are from the alfalfa, pea, bean, clover and the Leucaena groups of legumes. Rhizobia that are slow-growing are found in the soybean and cowpea groups of legumes.

Rhizobia from the various cross-inoculation groups appear similar on YMA or YM medium and host legumes from which they were isolated cannot be determined from their growth characteristics.

Recommendations to farmers from results of this demonstration

Knowledge on the growth characteristics of rhizobia is not essential to the farmer. Demonstration of growth characteristics of rhizobia is useful knowledge to the extension agent. When discussing legume BNF with researchers and inoculant producers, it is important that regional extension workers with expertise in BNF know some characteristics of rhizobia. CONDUCTING THE DEMONSTRATION

Pure cultures of rhizobia from different cross-inoculation groups are needed. This demonstration requires the assistance of a microbiologist with experience working with rhizobia. The demonstration can only be set up if laboratory facilities are available for sterile media preparation and equipment for performing sterile or aseptic work. See Methods in Legume Rhizobum Technology for procedures.