Authentication of rhizobia and assessment of the legume symbiosis in controlled plant growth systems

After strains of rhizobia have been isolated from nodules (Chapter 3), and (ide¬ally) before long-term preservation (Chapter 4), the strains should be examined to ensure they retain the essential features of nodule bacteria. The first step in this process is termed ‘authentication’, which examines the ability of the strain to in¬fect a legume to form a nodule. Following this, strains may be evaluated for their ability to fix nitrogen. This latter characteristic is sometimes termed ‘effectiveness’; it is an assessment of the genetic compatibility between the host plant and the rhizobium strain for nitrogen fixation. If a strain can nodulate a legume and fix N2 effectively in the glasshouse environment, the researcher may wish to proceed further, to assessment in the field. However, if the strain is to be released to the field, then ‘duty of care’ requires that we have an understanding of its host-range (Section 5.2). This is because releasing strains into the general environment that might be detrimental to existing legumes either agricultural or natural would be negligent. The techniques described in this chapter allow a researcher to compare strain symbiotic performance across a spectrum of plant genotypes to fulfil this duty of care.