CULTIVAR VARIATION IN TRAITS AFFECTING EARLY NODULATION OF COMMON BEAN

Phaseolus vulgaris L. is inferior to other grain legumes in nodulation and N2 fixation, with early-flowering cultivars particularly limited in the ability to satisfy their N needs through fixation. Extending the period of fixation could reduce the dependence of this crop on N-fertilization. We sought bean accessions that were early to nodulate, and tried to identify specific traits that contributed to this ability. A root-tip marking procedure was used to evaluate the earliness in nodulation of 40 accessions of common bean. With Rhizobium leguminosarum bv. phaseoli strain UMR1084 the accessions varied in uppermost nodule position relative to the root-tip mark (RTM; -2.05 mm to +6.12 mm), number of nodules produced above the RTM (0.57-3.93), and percentage of plants nodulated above this mark (37.7-97.1%). Cultivars nodulated earlier with strain UMR1084 and UMR18'' than with strain UMR1242. Accessions RIZ23, RIZ108, N80068, and WI21-58 nodulated rapidly with all strains tested, while 'Mantequilla tropical', RIZ21, and 0051 were slow to nodulate. Selected bean accessions were then examined for inoculum dose response with UMR1084, for the percentage of infection sites giving rise to nodules, for the rate of development of nodules in the period 16 to 25 days after planting (DAP), and for plant growth and accumulation of N at 51 DAP. In these studies, 'Puebla 152' showed excellent nodule development and growth to the 25 DAP harvest, while the experimental line RIZ21 accumulated significantly more N and grew better in the period 25 to 51 DAP. The differences between these two accessions could be exploited in breeding programs for enhanced nodulation and N2 fixation in common bean.