Improvement of germination and growth of tomato seeds under phenolic acid stress by humic acid-medium and trace element compounds

Allelopathic stress induced by phenolic acids poses a significant challenge to sustainable agricultural development. To alleviate this stress in tomatoes, this study synthesized humic acid-based compounds with medium and trace elements, including calcium (HA-Ca), magnesium (HA-Mg), copper (HA-Cu), zinc (HA-Zn), iron (HA-Fe), and manganese (HA-Mn). Their potential to alleviate phenolic acid stress during tomato seed germination were evaluated. Among them, HA-Ca exhibited the most pronounced mitigation effect, followed by HA-Mg, HA-Mn, HA-Fe, HA-Zn, and HA-Cu. Compared with the p -coumaric acid ( p -CA) stress treatment, HA-Ca application for 7 days significantly increased seed germination rate, germination index, root length, root surface area, and root volume by 15.4 %, 623.1 %, 530.4 %, 403.3 %, and 301.2 %, respectively. Additionally, HA-Ca enhanced root activity of tomato radicles by 105.8 %, elevated the activities of key antioxidant enzymes (SOD, POD, CAT, APX, GR) by 106.6 %–244.1 %, and raised the GSH/GSSG ratio by 185.5 %. Conversely, it reduced reactive oxygen species (H₂O₂, O₂·⁻, •OH) and malondialdehyde (MDA) levels by 55.8 %–90.3 %, and lowered osmotic adjustment substances (soluble sugar, soluble protein, proline) by 70.1 %–88.5 %. At the molecular level, HA-Ca down-regulated the expression of SlLOX2, SlFAD2–2, PAL and SlCBF1 by 44.0 %–96.8 %, while up-regulated SlMAPK3 and SlGRAS1 by 91.0 % and 31.0 %. Furthermore, the addition of HA-Ca facilitated a 99.9 % removal of p -CA from the culture medium, which was 62.1 % higher than that in the p -CA stress control, and increased the solution pH by 0.39 units. In conclusion, HA-Ca alleviates phenolic acid stress in germinating tomato seeds by improving the growth environment and enhancing physiological and antioxidant defenses.