Humic Acid Mitigates Drought Stress in Tomato

Drought stress, one of the most important abiotic stresses, severely limits global crop production. To increase tolerance to this stress, environmentally friendly practices are emphasised. Humic acid, one of the most important natural biostimulants, has positive effects on plant growth and yield. Recently, it has also been reported to play an important role in resistance to various abiotic stresses. However, many physiological and molecular mechanisms by which humic acid confers drought resistance have not been fully elucidated. Therefore, the effects of humic acid application on different morphological and physiological stress indicators and some antioxidative enzyme gene expressions of tomato seedlings under drought stress conditions were investigated in this study. It was found that drought stress decreased shoot fresh/dry weight, root fresh/dry weight, shoot and root length, chlorophyll and relative water content of plants by 67%, 56%, 31%, 38%, 22%, 20%, 15% and 25%, respectively. Humic acid application significantly increased these parameters, while reducing ion leakage, MDA and proline levels. The antioxidant enzyme gene expression of tomato seedlings under drought conditions showed no significant difference in SOD and APX gene expression, whereas CAT gene expression increased and GR gene expression decreased with humic acid application. Our results showed that humic acid application interacted with stress-related antioxidant enzyme gene expression and may be effective in reducing drought stress.