Optimal levels of N, P, and K for the cultivation of single-stemmed roses in a closed hydroponic system

It is necessary to identify the optimum levels of mineral nutrients for the commercial production of single-stemmed roses in a closed hydroponic system to avoid the accumulation of undesirable ions in the root zone and to improve shoot growth and flower quality. We determined the optimal concentrations of nitrogen (N), phosphorous (P), and potassium (K) in a nutrient solution by analyzing the amount of mineral nutrients absorbed by the plant, plant growth, and photosynthesis characteristics. Rose plants (Rosa hybrid L. cv. Red Velvet) were subjected to different concentrations of macronutrients, including N (0, 10, 50, 100, 150, or 200 mg·L⁻¹), P (0, 10, 20, 40, 80, or 100 mg·L⁻¹), and K (0, 10, 50, 100, 150, 200, or 250 mg·L⁻¹). The uptake of nitrate-N (NO₃ ⁻) was inhibited by high P levels, and the NO₃-N concentration in the nutrient solution was significantly higher in the 100 mg·L⁻¹ P treatment than in the other P treatments, indicating reduced NO₃ ⁻ uptake by the plants. The concentration of phosphate (PO₄ ⁻) decreased considerably in all N and K treatments during the entire growth period. To ensure a sufficient supply of P for single-stemmed roses, it is necessary to increase the PO₄ ⁻ concentration in the nutrient solution. Increasing the K⁺ level significantly increased the concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺) in the nutrient solution. This may be the result of low Ca and Mg absorption from plants over the growth period. Although there was an increase in the fresh weight and stem length with higher levels of N [NO₃ ⁻ and ammonium (NH₄ ⁺)], PO₄ ⁻, and K⁺, growth did not significantly increase at levels higher than 150 mg·L⁻¹ N, 40 mg·L⁻¹ P, and 200 mg·L⁻¹ K treatments. The photosynthetic rates (P ₙ) increased rapidly as the concentration in the nutrient solution increased to 100 mg·L⁻¹ N, 40 mg·L⁻¹ P, and 150 mg·L⁻¹ K, followed by a reduction as the ion concentrations increased to the maximum ranges. A distinct reduction of the P ₙ was observed in the plants supplied with nutrient levels above 150 mg·L⁻¹ N, 80 mg·L⁻¹ P, and 200 mg·L⁻¹ K. Considering the nutrient-water absorption patterns, growth, and photosynthesis characteristics of single-stemmed roses, the mineral nutrient levels of 100–150 mg·L⁻¹ N, 30-50 mg·L⁻¹ P, and 100–150 mg·L⁻¹ K were found to be optimal for the production of single-stemmed roses in a closed hydroponic system.