The biostimulant effects of humic substances (HS) in plants strictly rely on their specific properties. Therefore, in this work we evaluated the capacity of three humates (H1, H2, H3) supplied by the company NEOVA (Norway) and differing in chemical and structural properties to promote the growth and acquisition of phosphorus (P) by maize (Zea mays L.) plants grown in hydroponics with either high (250 M) or low (25 M) P concentration. We first determined the chemical and structural features of humates. Then, we investigated whether differences in P accumulation between plants treated with different humates were due to a specific action of these products on the activity of P mining enzymes (phosphatase and phytase) and on target genes. Our results indicate that H2 was the humate that most stimulated the production of the leaf and root biomass of maize, in both P conditions. However, all humates increased the SPAD index of plants grown with high P, which was indicative of increased chlorophyll content. H2 and H3 promoted nitrogen (N) accumulation in roots under low P, while this effect was observed for all tested products under high P. In leaves, H2 was the most effective in stimulating N accumulation, regardless of the amount of P supplied. The carbon (C) content was increased by humates only in roots, under high P. All three humates increased the concentration of P in leaves of maize under high P concentration. However, only H2 was successful in determining this effect at low P. No variation in root P content was observed between plants treated with humates and the control at low P, but an increase in P accumulation was evident in plants supplied with high P. Phosphorus acquisition efficiency (PAE) was higher in plants treated with humates compared to control plants at low P, but the maximum PAE values were determined for high P-plants treated with H2. In general, PAE values were greater in low P- than high P- plants. With respect to the P-recycling enzymes in roots, the activity of phosphatase was enhanced in plants added with H1 and H2 humates at low P. Maximum values of phosphatase activity were determined in roots of plants treated with H2. The same trend was observed in plants receiving the high P dosage. The activity of phytase was lower compared to phosphatase and overall, more pronounced at low P. As in the case of phytase, maximum values of phytase activity were recorded in maize plants treated with H2, in both P conditions. [...]
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