Phytochemical changes of Denak (Oliveria decumbens) in response to salinity stress and application of salicylic acid hormone

Salinity stress is one of the most important problems in the cultivation and production of agricultural products, including medicinal plants. The expansion of saline lands following continuous droughts in recent years increases the importance of the tolerance level of medicinal plants determination, their type of reaction, and the necessity of finding a solution to reduce the damage of mentioned stress on plants and increasing the productivity of the above lands.

To study the effect of salinity stress and salicylic acid hormone on biochemical traits of Denak (Oliveria decumbens), a factorial pot experiment was conducted as randomized complete block design with three replications in the research farm of Shahid Chamran University of Ahvaz in 2020-2021. The experiment factors included irrigation water salinity in four levels (1.1 (tap water), 2.5, 5, 7.5 dS/m) and foliar application of salicylic acid hormone in four levels (0, 100, 200, 500 mg/L-1). Biochemical traits included total soluble carbohydrates, proline, malondialdehyde, sodium and potassium elements, the activity of superoxide dismutase, catalase, guaiacol peroxidase, ascorbate peroxidase, polyphenol oxidase and phenylalanine ammonia lyase enzymes were measured at flowering stage.

The interaction effect of salinity stress and foliar application of salicylic acid hormone significantly changed all the traits of Denak. The foliar application of 200 mg/L-1 of this regulator in plants under 5 dS/m salinity stress increased the amount of total soluble carbohydrates, proline, the activity of superoxide desmutase, catalase, guaiacol peroxidase, ascorbate peroxidase, polyphenol oxidase enzymes compared to the control treatment. The highest level of activity of ascorbate peroxidase and phenylalanine ammonialyase enzymes was also obtained in the foliar spraying of plants under 5 dS/m salinity stress by the concentration of 500 mg/L-1 of salicylic acid hormone, but no significant difference was observed between this concentration and the application of 200 mg/L-1 of this hormone. The maximum concentration of potassium element and the minimum amount of malondialdehyde and sodium element of Denak leaf were also recorded in the treatment of plants under 2.5 dS/m salinity stress by the concentration of 200 mg/L-1 of salicylic acid hormone. The salicylic acid regulator as a very important messenger in the plant's defense system through increasing the stimulation of antioxidant enzymes in Denak reduced the negative effects of free radicals, membrane destruction and the production of malondialdehyde compound in a 2.5 dS/m salinity stress. On the other hand, the application of this hormone reduced the damage caused by the secondary stress of dehydration by increasing the ability of the plant to produce osmotic pressure regulating compounds including soluble carbohydrates and proline.

Due to its ability to increase the production of osmolyte compounds including soluble carbohydrates and proline, activation of antioxidant enzymes and maintaining the stability of cell membranes, Denak is able to withstand salinity stress up to the level of 5 dS/m. The application of 200 mg/L-1 of salicylic acid hormone is beneficial and recommended due to its positive effect on antioxidant capacity and increasing the production of osmotic protective compounds in the plant as a solution to increase the resistance of Denak against the salinity stress.