Effect of Molybdenum on Nitrogen Assimilation and Oxidative Stress on Phaseolus vulgaris seedlings

Abstract

Background: Molybdenum is an essential plant micronutrient that regulates nitrogen assimilation and redox metabolism, although its concentration-dependent effects in Phaseolus vulgaris seedlings remain insufficiently characterised. Aims: This study evaluated the concentration-dependent effects of molybdenum on nitrogen assimilation, nitric oxide metabolism, antioxidant defence, and oxidative stress responses in Phaseolus vulgaris L. seedlings. Study Design: A controlled growth-chamber experiment was conducted using four molybdenum concentrations: 0, 0.001, 0.1, and 1 mM. Place and Duration of Study: Phaseolus vulgaris L. cv. Rajmah seedlings were grown for 10 days under controlled environmental conditions in acid-washed sand supplied with half-strength Hoagland nutrient solution. Methodology: Seedling growth and biochemical responses were evaluated following molybdenum treatment. Nitrogen assimilation was assessed by measuring nitrate reductase and glutamine synthetase activities. Nitric oxide content and the activities of major antioxidant enzymes, including superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidase, were determined in leaf and root tissues. Protein, proline, and chlorophyll contents were also analysed as indicators of metabolic and stress responses. Results: Molybdenum produced a concentration-dependent biphasic response. Seed germination, shoot growth, and seedling vigour increased up to 0.1 mM molybdenum, which was identified as the optimum treatment, whereas 1 mM adversely affected seedling growth. Molybdenum significantly modulated nitrate reductase-associated nitrogen metabolism and nitric oxide levels in a tissue- and concentration-dependent manner. Glutamine synthetase-specific activity was highest at 0.001 mM and declined at higher concentrations. Antioxidant enzymes showed differential responses in leaves and roots. At 1 mM molybdenum, growth inhibition, together with increased root proline accumulation, indicated the development of oxidative and osmotic stress. Conclusion: Moderate molybdenum supplementation improved growth and regulated nitrogen assimilation and redox metabolism in P. vulgaris seedlings, with 0.1 mM being the most favourable concentration under the experimental conditions. Excess molybdenum disrupted metabolic homeostasis and activated stress-related defence mechanisms.