Interaction among growth cycle, photosynthesis, and nodule enzymatic activity in Phaseolus vulgaris L. inoculated with Rhizobium tropici CIAT

Abstract

Common bean ( Phaseolus vulgaris ) exhibits lower rates of biological N ₂ fixation (BNF) than other legumes and often relies on nitrogen (N) fertilizers to maintain yields. Since BNF is supported by photosynthesis, there is ongoing debate about whether super‐early‐cycle cultivars can support the photosynthetic costs associated with BNF. We hypothesized that inoculation with an efficient Rhizobium strain would directly enhance nodule activity and BNF and indirectly stimulate photosynthesis across common bean cultivars. A greenhouse experiment was conducted using three cultivars with different growth cycles: BRS FC104 (super‐early, 65 days), BRS Pontal (intermediate, 87 days), and BRS Vereda (long, 93 days). Plants were subjected to one of three treatments: (1) no inoculation or N fertilization, (2) N fertilization without inoculation, or (3) inoculation with Rhizobium tropici CIAT 899. At growth stages V4 and R6, we measured photosynthesis rates, shoot and root biomass, petiole N‐ureides, leaf N content, soluble sugars, and nodule enzyme activities (GS and GOGAT). Inoculation increased nodule enzymatic activity (e.g., GOGAT at R6: 6.8 μmol NADH min ⁻¹ in inoculated plants, compared with 5.3 in control and 4.8 in N‐fertilized plants), indicating enhanced BNF. The intermediate‐cycle cultivar BRS Pontal showed the highest N‐ureide accumulation, even under N fertilization (37.7 nmol g ⁻¹ , compared with 15.2 in BRS FC104 and 17.0 in BRS Vereda). Thus, BRS Vereda could serve as a model for future studies aimed at improving BNF in common beans. Under the conditions of this study, super‐early cycle BRS FC104 sustained BNF at levels comparable to those observed in the intermediate‐ (BRS Pontal) and long‐cycle (BRS Vereda) cultivars. Overall, inoculation increased both photosynthesis and nodule activity, regardless of growth cycle length.