N2‐Fixation in Field Beans Quantified by 15N Isotope Dilution. II. Effect of Cultivars of Beans1

Abstract

AbstractField beans (Phaseolus vulgaris L.) have been considered inferior in nis (nitrogen fixation supportive trait), i.e., in their ability to support and benefit from their symbiotic association with N2‐fixing Rhizobium phaseoli. This paper reports the use of N balance in phytotron studies and 15N isotope dilution in the field to determine the effect of R. phaseoli strains and mineral N levels on the expression of nis in 26 cultivars of field beans. Evaluation for nis under N‐free conditions in the phytotron forced total dependence on seed and atmospheric N and provided a useful screening technique before field experimentation. Ranking of cultivars for nis was highly dependent on R. phaseoli strain, mineral N levels, growth habit, growth stage, and temperature regime during evaluation. R. phaseoli strains 3644 and 8215 resulted in higher N2 fixation with temperate beans while strain 3605 was unique in high N2 fixation with two tropical cultivars, Cargamantn and Sangretoro. The addition, in the field, of 40 kg fertilizer N ha−1 on a Typic Haploboroll soil caused a 10% reduction in percent N derived from atmosphere (% Ndfa) in most cultivars but had no effect on ‘Redkloud’. In contrast, ‘Limelight’ suffered a 60 % reduction in % Ndfa. This indicated a host‐specific reaction in nis to mineral N and potential for breeding this resistance into other bean lines. Both in the phytotron and the field, climbing bean cultivars had higher % Ndfa and thus superior nis than bush cultivars. Some cultivars were superior when evaluated at anthesis but not at maturity, indicating a difference in the N2‐fixing span of the cultivar. In the field, % Ndfa of beans was approximately 50 %, the other 50 % being derived from fertilizer and or soil N. The actual amounts of Nz fixed varied between 40 kg ha−1 and 125 kg ha−1 depending on the cultivar. Thus, R. phaseoli may he considered as efficient as R. japonicum is with soybeans in supplying atmospheric N2 to the plant.