Increasing the nitrate-to-ammonium ratio improved plant growth and nitrogen uptake in pineapple seedlings

Introduction: Previous studies indicate that the combined supply of both nitrate nitrogen (NO₃⁻) and ammonium nitrogen (NH₄⁺) is more beneficial for pineapple growth than the exclusive supply of either NO₃⁻ or NH₄⁺ alone. However, the optimal ratio of NO₃⁻:NH₄⁺ that maximizes pineapple growth remains to be fully elucidated, which limits the optimization of nitrogen fertilizer management in pineapple cultivation. Methods: L. cv. Comte de Paris were subjected to hydroponic experiments with five NO₃⁻:NH₄⁺ ratios (0:100, 30:70, 50:50, 70:30, and 100:0) and a fixed total nitrogen concentration of 4 mM. The shoot and root growth characteristics, as well as carbon (C) and nitrogen (N) accumulation traits of pineapple plants, were systematically investigated. Additionally, the NO₃⁻ and NH₄⁺ uptake rates of roots were determined. Results: Compared with other treatments, the NO₃⁻:NH₄⁺ ratio of 70:30 significantly increased total root length and surface area, and exhibited the highest NO₃⁻ uptake rate. With increasing nitrate proportions in the nitrogen supply, plant N uptake and the contents of chlorophyll a (Chl a) and chlorophyll b (Chl b) were significantly increased. Specifically, relative to the sole NH₄⁺ treatment (0:100), the 70:30 ratio treatment increased leaf area by 61.10%, leaf dry matter content by 24.40%, plant N accumulation by 19.49%, and plant C accumulation by 21.62%. Enhanced N uptake and photosynthetic pigment synthesis facilitated leaf area expansion, thereby promoting overall plant growth. Discussion: These results indicate that a NO₃⁻:NH₄⁺ ratio of 70:30 is optimal for pineapple growth, as it effectively enhances both root and shoot development and improves C and N accumulation. This finding provides valuable insights for optimizing nitrogen fertilizer management strategies, which is crucial for promoting pineapple productivity in practical cultivation.