Effect of low pH on nitrogen fixation of common bean grown at various calcium and nitrate levels

Abstract

The influence of low pH (4.0–4.5) of the medium on nodulation, nodule biomass, nodule nitrogenase activity, and ultrastructure at various concentrations of calcium (Ca) (0.127 mM and 0.508 mM) and nitrate (NO₃) (0.390 mM and 1.560 mM) at different stages of development (2nd, 3rd, 6th, and 7th compound leaf) of the common bean plants was studied. The experiment was conducted using Hellriegel's nutrient solution under greenhouse conditions. Bean root nodule nitrogen (N₂) fixation was influenced favorably at a decrease of Ca (0.127 mM) and NO₃ (0.390 mM) concentrations in the growth solution. In contrast, the biomass of the roots, stems, leaves, and pods decreased at low Ca and NO₃ levels while the most sensitive were the leaves and pods of the plants. The number of nodules, their fresh and dry weight, nitrogenase activity, and ultrastructure was affected to a greater extent at a low pH of the medium when Ca and NO₃ concentrations were low (0.127 mM and 0.390 mM, respectively). Under these conditions, the roots, stems, leaves, and pods were slightly affected. Nodule N₂ fixation was more sensitive to low pH at the 2nd and 3rd leaf stage of plant development. The root and pod biomass had the same tendency of change as the nodules. On the contrary, at low pH and decreased Ca and NO₃ levels (0.127 mM and 0.390 mM, respectively), the stem and leaf biomass increased significantly at the later stages of bean plant development (6th and 7th leaf). Our conclusion was that the stress effects of low pH of the medium had different affects at various Ca and NO₃ concentrations in the nutrient solution and stages of common bean plant development.