Salinity stress mitigation by radicular and foliar humic substances application in lettuce plants

Salinity stress constitutes one of the main abiotic stresses that considerably reduces crop yield. An approach to enhance plant growth under salt stress involves the addition of humic substances (HS) to roots or leaves. Here, we evaluated the potential use of BLACKJAK®, an HS-based product, to enhance salt tolerance in lettuce (Lactuca sativa L.). For this aim, plants were exposed to salinity (100 mM NaCl), salinity + HS: radicular (R)-HS (0.40 mL/L and 0.60 mL/L) and foliar (F)-HS (7.50 mL/L and 10.00 mL/L), along with a control (without NaCl). Parameters related to plant growth, Na+ and K+ accumulation, photosynthetic activity, oxidative stress, enzymatic and non-enzymatic antioxidants, as well as proline levels were evaluated. Results showed that R and F-HS considerably enhanced salinity tolerance. In addition, F-HS offered a greater improvement of plant growth in relation to shoot fresh weight, shoot relative growth rate, and foliar area, being 10.00 mL/L the best dose. This tolerance could be associated with reduced Na+ translocation to the shoot and enhanced shoot K+ accumulation, decreasing Na+/K+ ratio. Furthermore, HS improved the net photosynthetic rate, Rubisco carboxylation efficiency, and photosystem II performance, and reduced ROS levels and lipid peroxidation. Hence, our data show the potential use of BLACKJAK® to improve lettuce tolerance to salinity, with foliar application slightly better than radicular to achieve this aim, especially at 10.00 mL/L dose.