Leonardite-derived humic substances enhance phosphorus use efficiency in lettuce through integrated physiological and metabolic reprogramming.

Phosphorus (P) is essential for plant growth, and its deficiency significantly limits crop productivity. Plant biostimulants offer a sustainable approach to enhance plant tolerance to nutrient stress and reduce fertilizer use. Among these, humic substances (HS) derived from leonardite show promise, though their physiological effects under P deficiency remain unclear. This study evaluated the impact of a leonardite-derived HS on lettuce (Lactuca sativa L.) grown under two P regimes: high P (HP, 1 mM) and low P (LP, 0.2 mM). HS were applied at two concentrations via root (R1: 0.40; R2: 0.60 mL L-¹) or foliar (F1: 7.50; F2: 10.00 mL L-¹) treatments. Phosphorus shortage reduced shoot biomass and chlorophyll content, but stimulated root growth, total phenolic content (TPC), and antioxidant capacity. HS improved shoot growth across both P levels, with R2 being most effective at HP. Under LP, both HS doses produced similar effects. The enhanced tolerance to P deficiency conferred by HS was associated with improved root biometric traits, enhanced P acquisition (PAE), P utilization (PUtE), and overall P use efficiency (PUE), along with elevated acid phosphatase activity, chlorophyll, glycine betaine, TPC, and antioxidant capacity. All HS treatments enhanced root elongation; notably, F2 treatment increased root volume and diameter under LP. Furthermore, HS application modulated the leaf phenolic profile: F2 enhanced the accumulation of specific phenolics, while root-applied HS increased TPC and antioxidant response under LP. In conclusion, this leonardite-derived biostimulant demonstrated potential to enhance plant performance under P-limited conditions, supporting its use for more sustainable nutrient management.