Protective Effects of Humic Acid on Growth and Photosynthetic Pigment Content of Tomato Plants (Solanum lycopersicum L.) under Salt Stress

Abstract

Salinity is an important abiotic stress factor that restricts tomato growth and productivity, particularly in arid and semi-arid regions. This study evaluated the effects of foliar humic acid application on the growth and photosynthetic pigment responses of tomato plants exposed to salt stress. Tomato plants (Solanum lycopersicum L. cv. Falcon) were grown under greenhouse conditions in a randomised complete block design with four replications. Four treatments were applied: control, 100 mM NaCl, 200 ppm humic acid, and 100 mM NaCl combined with 200 ppm humic acid. Treatments were applied once weekly for eight weeks. Plant height, plant fresh and dry weight, root fresh and dry weight, root length, chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents were determined. Salt stress reduced plant fresh weight by 17.4%, plant dry weight by 11.7%, root length by 10.0%, and total chlorophyll content by 49.4% compared with the control. Humic acid application under salt stress improved several growth parameters compared with salt stress alone. In the combined salt and humic acid treatment, plant fresh weight increased by 13.2%, plant dry weight by 23.3%, root fresh weight by 29.4%, and root length by 14.0% compared with the salt-stressed group. Total chlorophyll content was also numerically higher under the combined treatment than under salt stress alone, although this difference was not statistically significant. Correlation analysis showed a strong positive relationship between chlorophyll a and chlorophyll b. Multivariate analyses indicated that salt-stressed plants differed from control and humic-acid-treated plants, while humic acid partially moderated the salt-stress response. These findings suggest that 200 ppm foliar humic acid may help reduce some adverse effects of salt stress in tomato plants under greenhouse conditions.