Nitrogen Regulates the Concentration and Accumulation of Macronutrients in Vegetative and Reproductive Organs of Mexican Marigold (Tagetes erecta L.)

Nitrogen (N) is a key macronutrient that influences the uptake and partitioning of other essential elements in plants. In this research, we evaluated the effect of different N concentrations in the nutrient solution (0, 4.2, 8.4, and 12.6 mg L−1) during the flowering stage on the concentration and accumulation of macronutrients in organs of Mexican marigold (Tagetes erecta L.) ‘Inca’. After 40 days of treatment, plants were separated into leaves, flowers, stems, and roots to determine the concentrations of N, P, K, Ca, Mg, and S, as well as their accumulation based on dry biomass. Nitrogen supply significantly affected dry biomass production and its partitioning among organs, promoting biomass allocation to leaves and flowers while reducing relative root biomass at higher N concentrations. Nitrogen concentrations and accumulation increased in leaves, stems, and flowers as N supply increased, whereas an inverse relationship was observed in roots. When applying 8.4 and 12.6 mg N L−1, phosphorus displayed enhanced concentrations in leaves and stems, although root tissues did not change the concentration of this nutrient. When N was supplied at up to 8.4 mg L−1, the concentration of potassium rose in aboveground organs but decreased at the highest dose, while its accumulation in roots was reduced under high N concentrations tested. Calcium exhibited greater accumulation in the aboveground organs, particularly at 12.6 mg N L−1. Magnesium concentration and accumulation increased in aboveground organs with increasing N supply, whereas its accumulation in roots decreased. The highest concentrations of sulfur in leaves and stems were observed at 8.4 mg N L−1, and its accumulation in the aboveground organs tended to stabilize at the highest dose. Effect size analysis (partial ηp2) revealed that N supply explained a large proportion of the variance in macronutrient concentration and accumulation in aerial organs, whereas responses in roots were generally weaker and nutrient specific. Overall, our data indicate that intermediate N levels (8.4 mg L−1) boost a more efficient nutritional balance in the aboveground organs, while the highest dose predominantly enhances Ca and Mg accumulation. Understanding how these plants respond to nitrogen can help improve the quality of Mexican marigold crops and make better use of fertilizers.