Improving lucerne nodulation at low pH: contribution of rhizobial and plant genotype to the nodulation of lucerne seedlings growing in solution culture at pH 5

The effect of rhizobial strain and lucerne genotype on the nodulation of lucerne seedlings growing in solution cultures maintained at pH 5 was measured in two greenhouse experiments. Sinorhizobium strain SRDI291 increased the percentage of plants nodulated when compared with the commercial inoculant strain RRI128 (89% cf. 45% and 86% cf. 23% in the two experiments, respectively). Similarly, the mean number of nodules per nodulated plant (NNP) was greatest in the SRDI291 treatments (3.5 cf. 1.7 and 3.1 cf. 1.8). Plant genotype had fewer, but still significant effects on nodulation. Overall, lucerne genotype CRCSA29 had the highest percentage of plants forming nodules and most NNP (63% and 2.9); genotype L97a had the lowest (46% and 1.9). Greater variation in NNP occurred when the lucerne genotypes were inoculated with RRI128. Most notable was the better nodulation of CRCSA29 with 2.8 NNP compared with only 1.2 NNP for the two unselected genotypes (SARDI Ten and L97a). The lucerne genotypes typically had variable NNP, as exemplified in CRCS Lambda 29 inoculated with RRI128 where it ranged from 0 14, providing the opportunity for further plant selection. The work demonstrates that there remains substantial opportunity to increase the potential of lucerne to nodulate at low pH. Gains would appear to be most easily made by changing the strain of rhizobia that is used for inoculation.