Humic Acid Alleviates Low-Temperature Stress by Regulating Nitrogen Metabolism and Proline Synthesis in Melon (Cucumis melo L.) Seedlings
Libao Zhu, Haihe Liu, Yanping Zhang, Yanxia Cao, Y. Hu, Yalun Wang, Haiqiang Zheng, Mengze Liu
Abstract
Melon is a thermophilic crop that is extremely sensitive to temperature changes. Humic acid (HA) is an eco-friendly biostimulant that enhances plants against abiotic stresses. However, the effect of HA on the cold tolerance of melon plants under low-temperature scenarios is still unclear. This study aimed to find out the effects of HA treatment on the morphological, physiological, and nitrogen metabolism of melon seedlings under low-temperature stress. HA treatment significantly enhanced plant height, stem diameter, fresh weight, dry weight, chlorophyll content (up to 33.17%), maximum photochemical efficiency (Fv/Fm), root architecture, superoxide dismutase (SOD), and catalase (CAT) activity. HA also promoted the degradation of nitrate nitrogen (NO3−-N); the synthesis of ammonium nitrogen (NH4+-N), free amino acids, proline, and soluble protein; and nitrogen metabolism (NR, GS, GOGAT, and GDH, up to 181.83%) and proline-related enzyme activity (P5CS and OAT, up to 81.97%). HA significantly increased the expression of nitrogen metabolism and proline metabolism genes. In summary, HA alleviated the damage caused by low-temperature stress by improving levels of antioxidant enzymes, nitrogen metabolism, and proline synthesis.