Reduction of Ammonium Toxicity by Potassium in Hydroponically Cultivated Alfalfa (Medicago sativa): Growth and Physiological Responses

Alfalfa (Medicago sativa) typically thrives on nitrate nutrition, but the environmental impact of nitrate overuse necessitates alternative nitrogen sources. While ammonium (NH₄⁺) is a promising candidate, it often induces toxicity in plants. This study investigates the potential of potassium (K⁺) supplementation to mitigate NH₄⁺ toxicity in hydroponically grown alfalfa over a 17-day period. Plants were subjected to varying nitrogen regimes: a nitrate control (3 mM NO₃⁻), and two NH₄⁺ concentrations (3 and 6 mM) combined with three K⁺ levels (0.2, 1.2, and 3 mM). A combination of 6 mM NH₄⁺ and 3 mM K⁺ proved lethal, causing complete plant mortality. In contrast, the specific regimen of 3 mM NH₄⁺ with 3 mM K⁺ significantly alleviated toxicity, restoring plant biomass to approximately 50% of the nitrate control. This mitigation was linked to a significant reduction in toxic NH₄⁺ accumulation in shoots and a 2.3-fold increase in K⁺ uptake compared to low-K⁺ treatments. Physiologically, this optimal treatment maintained chlorophyll content, soluble protein, and polyphenol levels comparable to control plants, while also boosting antioxidant activity by 35%. These results demonstrate that a balanced NH₄⁺/K⁺ nutrition, specifically at a 1:1 ratio (3 mM each), can effectively mitigate ammonium toxicity by regulating its uptake and translocation, preserving photosynthetic function, and enhancing antioxidant capacity. This strategy offers a sustainable approach for incorporating ammonium-based fertilizers in alfalfa crop.