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

Abstract

Abstract Field 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 N 2 ‐fixing Rhizobium phaseoli. This paper reports the use of N balance in phytotron studies and 15 N 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 N 2 fixation with temperate beans while strain 3605 was unique in high N 2 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 N 2 ‐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 N 2 to the plant.