Nitrate-dominated nutrition delays bolting and improves yield of hydroponic lettuce: interactive roles of ammonium:nitrate ratios and salicylic acid

Optimizing the ammonium-to-nitrate ratio in the nutrient solution can enhance lettuce growth performance, yield, and quality by improving nitrogen uptake, assimilation, and utilization. Delayed bolting is highly desirable in lettuce production as it allows prolonged vegetative growth and improved marketable yield. Agronomic strategies that suppress or delay flowering may improve the efficiency of lettuce cultivation. Salicylic acid (SA) is a plant signaling molecule reported to stimulate growth and influence flowering behavior in various crops. This study evaluated the interactive effects of varying ammonium:nitrate (NH₄⁺:NO₃⁻) ratios and SA concentrations on growth characteristics and bolting time of hydroponically grown lettuce. The experiment consisted of three NH₄⁺:NO₃⁻ ratios (100% NO₃⁻:0% NH₄⁺, 80% NO₃⁻:20% NH₄⁺, and 40% NO₃⁻:60% NH₄⁺) combined with four SA concentrations (0, 0.75, 1.5, and 3.0 mM). Data relating to head fresh weight and diameter, total plant biomass, stem length, diameter and biomass, root length and biomass, total soluble solids, chlorophyll index, and the number of days to bolting were recorded. All measured traits (except total soluble solids) were significantly influenced by nitrogen form, while SA effects were limited and trait dependent. Nitrate-dominated nutrition (100% NO₃⁻) delayed bolting by 5–6 days and increased head fresh weight by ~ 42% compared to NH₄⁺-containing treatments. A late-season heatwave likely accelerated floral initiation, potentially masking the full effect of SA on bolting regulation. For this reason, future experiments should be conducted under controlled conditions to more precisely assess the role of SA.