Multiple mechanisms are involved in the alleviation of ammonium toxicity by nitrate in cucumber (Cucumis sativus L.)

Ammonium (NH₄⁺) toxicity in plants might occur when supplied with NH₄⁺ as the sole nitrogen source or when exposed to high NH₄⁺ levels. The mechanisms behind NH₄⁺ toxicity and the potential of nitrate (NO₃^–) in mitigating the harmful effects of NH₄⁺ are still poorly explored.

Cucumber plants (Cucumis sativus L. cv. Maximus) were cultivated in a pH-buffered medium under control (Hoagland solution containing 14 mM NO₃^– and 2 mM NH₄⁺) or high NH₄⁺ concentrations (16 and 25 mM) without or with 1.0 mM NO₃^– (as KNO₃) for two weeks in a growth chamber.

Toxicity symptoms characterized by localized chlorotic patches coincided with the sites of H₂O₂ accumulation and damaged reaction centers of photosystem II. When NO₃^– was supplemented, the leaf symptoms were diminished or eliminated. Nitrate treatment increased leaf ascorbic acid, carotenoids, flavonoids, and anthocyanins and enhanced the activities of antioxidant enzymes. The roots displayed a high toxicity tolerance with an efficient NH₄⁺ assimilation capacity, and maintained carbohydrates level which was further enhanced by NO₃^–. Reduction of lignin under NH₄⁺ toxicity and its subsequent increase by NO₃ suggest that lignin may play a role in plant responses to NH₄⁺ and NO₃^–.

Lack of growth reduction in the roots but severe NH₄⁺ toxicity symptoms in the leaves leading ultimately to plant death indicates that the toxic effects of NH₄⁺ primarily target chloroplasts. The extensive biochemical adaptations induced by NO₃^– at much lower concentrations than NH₄⁺ suggest a signaling function for NO₃^–.