An Alternative to Urea Overdosage: Positive Effects of Urea–Ornithine Combined Treatment on Yield and Metabolism in Wheat

Although ornithine is a key intermediate in nitrogen metabolism and a precursor of polyamines, its potential to replace high-dose urea-induced phytotoxicity has not been previously studied. To investigate whether ornithine reduces the negative effects of high-concentration nitrogen supply, the effects of standard urea dose (0.33 M) were compared to double dose (0.66 M) and combined urea + ornithine (0.33 M + 0.33 M) as seed priming or foliar application. Although seed-soaking with urea + ornithine had no positive effect, foliar application of ornithine modulated the effects of urea at physiological and metabolite levels. Deep metabolomic analysis revealed that double urea induced a metabolic crisis, especially while it accelerated polyamine catabolism; the urea + ornithine treatment countered this effect by stabilizing polyamine metabolism. This metabolic shift under the combined treatment redirected excess nitrogen from catabolism to polyamine synthesis and conjugation, and also preserved the antioxidant flavonoid pool. Additionally, double urea also disrupted the phenylpropanoid pathway, inducing hormonal stress signalling, and triggered senescence. The present study firstly provides information about the modulatory role of ornithine on foliar urea fertilization of wheat. Ornithine induces a metabolically primed state, which metabolic shift may support efficient nitrogen remobilization before and during grain filling; in addition, it could enhance plant adaptability to subsequent environmental stresses. The results may contribute to the development of sustainable nutrient management practices.