Introduction: Mentha spicata is one of the most widely consumed vegetables that is grown commercially worldwide. The essential oil and extracts of Mentha spicata are used in the cosmetics, food, and pharmaceutical industries. Salinity is one of the most important abiotic stresses that endanger plant growth and productivity. Salinity stress reduces the plant's capacity to absorb water, creates ion imbalances, and induces oxidative stress in the plant due to the accumulation of ions such as sodium and chlorine at toxic levels in cells and tissues. Salicylic acid plays an important role in improving physiological activities and increasing plant resistance to biotic and abiotic stress factors. Chitosan as a biostimulant can improve plant growth and yield. Furthermore, the combined application of salicylic acid and chitosan is believed to ameliorate the salinity defects more efficiently. This study aimed to investigate the application of chitosan-salicylic acid nanocomposite under salinity stress on the growth and some physiological traits of Mentha spicata in hydroponic culture.
Matrrials and Methods: To investigate the effects of different levels of salinity stress and application of chitosan-salicylic acid nanocomposite on the morphological and physiological traits of M. spicata, a factorial pot experiment based on a completely randomized design with four replications was performed at the University of Maragheh, Iran during 2019. Treatments used in this experiment included salicylic acid (1 mM), chitosan (10 mg L^-1), an aqueous mixture of chitosan and salicylic acid (1 mM and 10 mg L^-1), and 1% w/v chitosan-salicylic acid nanocomposite.
Results and Discussion: Salinity reduced the height of M. spicata compared to the control plants. However, foliar application of chitosan-salicylic acid nanocomposite improved plant growth and increased plant height under non-salinity conditions and different salinity levels compared to the controls. Salinity stress significantly reduced the fresh and dry weight of the M. spicata. The highest leaf chlorophyll index (56.9) was related to the treatment of 1% w/v of the chitosan-salicylic acid nanocomposite. The highest amount of proline (20.6 µmol g^-1 fresh weight) was related to the treatment of an aqueous mixture of chitosan and salicylic acid at a salinity level of 100 mM sodium chloride. The highest content of malondialdehyde (2.93 nmol g^-1 fresh weight) was obtained in the treatment without foliar application and a salinity level of 100 mM. The highest protein content (1.81 mg g^-1) was related to the treatment with 1% chitosan-salicylic acid nanocomposite and the treatment without salinity stress. Otherwise, the lowest amount (0.858 mg g^-1) for protein content belonged to the control and salinity of 100 mM. With increasing salinity levels, H₂O₂ content in the plant tissues increased. So that, the highest content for H₂O₂ was observed in the treatment without foliar application and salinity level of 100 mmol and, the lowest H₂O₂ content was recorded in the control treatment (without foliar application and salinity).
Conclusions: The overall results revealed that salinity stress had a detrimental effect on the growth and some physiological traits of the Mentha spicata L., but the foliar application of salicylic acid and chitosan alone or in combination had a promising role in reducing the effects of salinity stress on the M. spicata. Considering the progressive salinity stress incidence in many parts of the world and Iran, it seems that the results of the present study can be proposed to the agricultural extension sections to use the resulting mixture in plant breeding programs under saline areas to possibly reduce the salinity adverse effects.