Humic acid protects barley against salinity

Barley (Hordeum vulgare, cv. Radegast) plants cultured in Hoagland solution were exposed to NaCl and/or humic acid (HA) for 7 days. Plants revealed relatively high sensitivity to NaCl (100 mM), which was manifested by a considerable decline in growth, tissue water depletion, and high sodium accumulation. HA typically increased the content of organic metabolites (syringic acid, alanine, proline, ascorbic acid, glutathione, and phytochelatin 2), NaCl evoked the opposite effect (not for proline), and the combined treatment (NaCl + HA) showed mostly the positive impact of HA. However, these responses differed between shoot and root tissues. Salinity, but not HA, depleted the Krebs cycle acids (except for succinic acid). Salinity induced ROS formation, and HA reversed these symptoms, as evidenced by fluorescence microscopy. Changes of nitric oxide level were also detected. HA suppressed the NaCl-induced increase in Na, while the impact on other nutrients was not extensive. Moreover, foliar and hydroponic HA application revealed similar mitigating effects on NaCl stress. Overall, these data indicate the potential of HA to protect barley against NaCl stress by limiting Na uptake and positively impacting amount of some metabolites.