Biostimulant action of humic substances on tomato physiology and metabolism under water and nitrogen stresses

• Lignite-derived humic substances (HS) had positive and unique biostimulant effects • Water stress reduced plant water status, while HS mitigated these adverse effects • Nitrogen (N) stress decreased root development and N status, while HS reversed them • HS reduced enzymatic antioxidant activities but increased non-enzymatic antioxidant • HS application played important roles at the reproductive stage of plant N assimilation Humic substances (HS) have been widely tested as plant biostimulants for their bioactive effects on plant growth. However, despite the commonly used liquid HS products as foliar spray or solution injection, solid HS products used as media amendments to mitigate abiotic stress are less reported. This study investigated the mode of action of lignite-derived solid HS when mixed with peat substrate on mitigating tomato ( Solanum lycopersicum L. cv. Micro-Tom) responses under water (80% and 40% water holding capacity) and nitrogen (N, 100% and 40% recommended N input) stress across vegetative-, reproductive-, and whole-stage periods. HS-treated plants demonstrated significantly greater shoot and root development before, during, and after vegetative-, reproductive-, and whole-stage stress, resulting in higher biomass, fruit number, and fruit yield with ranges from 17% to 33%. Water stress reduced leaf water content, gas exchange activities, and water potential, while the application of HS mitigated these adverse effects. N stress inhibited root elongation during stress periods, decreased plant nitrogen level, reduced plant biomass and yield at the end of the study, and increased reactive oxygen species (ROS), membrane damage (MDA), and abscisic acid (ABA) levels during stress, especially when combined with water stress. HS reduced cell damage and oxidative stress markers (ROS, MDA) while also decreasing enzymatic antioxidant activities (SOD, CAT) and increasing non-enzymatic antioxidants (proline, carotenoids) and related signals (ABA) under both water and N stress conditions. HS application played important role at the late stage of plant N assimilation, producing higher amino acids, protein, and total N accumulation. Overall, lignite-derived HS effectively enhanced stress defense, photosynthesis, and nutrient use, demonstrating strong potential as a soil or media amendment to improve vegetable production under water and nitrogen stress.