Nitrogen absorption kinetics and accumulation of soluble nitrogenous fractions and sugars in sunflower

Abstract

Sunflower (Helianthus annuus L.) has emerged as a promising crop among oilseed species with agro-energetic importance. Considering the nutrients required by the sunflower crop, nitrogen is the most limiting for yield, besides affecting its oil production. This study aimed to evaluate metabolism aspects and nitrogen absorption kinetics (nitrate and ammonium), in sunflower (BRS 324) grown under different levels of this nutrient in nutritious solution, under controlled conditions, in a growth chamber. Plants were submitted to two nitrogen concentrations (0.2 mmol L^-1 and 2.0 mmol L^-1 of N-NH₄⁺ or N-NO₃^-), in a completely randomized experimental design. The kinetics absorption parameters (K_m and V_max), fresh weight and accumulation of soluble nitrogen fractions and sugars were evaluated. Plants grown in 2.0 mmol L^-1 of N-NH₄⁺ showed a higher V_max, indicating a greater capacity for transporting this ion. The stem presented a preferential accumulation of N-NO₃^- and soluble sugars, acting as the main site of energetic reserve of the plant. Treatments with NH₄⁺ showed negative correlations between fresh weight and amount of this compound in the plant tissue, which may be responsible for symptoms of toxicity, resulting in a reduction in the root/shoot ratio.