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.
Atero‐Calvo et al. (Thu,) studied this question.