Manganese (Mn) is crucial as a trace element for plant metabolism, but high concentrations in soil can induce symptoms of toxicity. This study aimed to evaluate the metabolism of nitrogen compounds and biomass production in Canavalia ensiformis and Cajanus cajan, clarifying the effects of this metal on nitrogen metabolism. The soil was treated with Mn concentrations of 80, 100, 120, 140, and 160 mg kg⁻¹. Differential responses were observed in the metabolism of nitrogenous compounds between species. C. cajan affected nitrogen metabolism in shoots, roots, and nodules, with significant variations in amino acids, total soluble proteins, ureides, and root biomass concentration. In contrast, C. ensiformis showed stability in the concentrations of compounds, mainly ureides, and proteins, even with increasing doses of Mn in the soil. These findings highlight the importance of nitrogen metabolism in legumes studied as a key aspect for understanding their Mn tolerance mechanisms in soil.

The use of nickel (Ni) as a fertilizer remains a topic of debate, particularly in non-legume species, as Ni is required only in trace amounts for optimal plant function. Urea application in plants, whether foliar or root-based, relies on the urease enzyme to convert urea into NH_4⁺ and CO₂, with Ni serving as an essential cofactor. In this study, we conducted an experiment using a 2 × 2 factorial design, combining two urea concentrations [4% and 8% (w/v)] with the absence or presence of Ni (0.3 g L⁻¹ supplied as NiSO₄·6H₂O). Gas exchange parameters were measured two days after fertilization. We quantified urease enzyme activity, urea content, photosynthetic pigments, carbohydrates, and other nitrogenous metabolites. The presence of Ni during foliar urea fertilization significantly increased the photosynthetic rate and photosynthetic pigments, which we attributed to improved urea assimilation. The combination of urea and Ni enhanced urease activity, leading to higher levels of various nitrogenous metabolites. Ni positively influenced foliar urea assimilation, promoting its conversion into organic compounds, such as proteins, while mitigating the toxic effects associated with urea accumulation.

1. Trace elements (TEs) are described in the literature as trace metals, minor elements, or potentially toxic elements (Hossain et al. 2021). Some TEs, including boron (B), copper (Cu), iron (Fe), ...