Effects of salicylic acid on physiological and phytochemical parameters of roselle (Hibiscus sabdariffa L.) under water shortage stress

Hibiscus sabdariffa L., commonly known as Roselle, is a plant cultivated in many regions of the world regions. Roselle has been used as a medicinal herb in traditional medicine for controlling and treating different diseases and physical ailments. These therapeutic effects are attributed to active compounds such as carotenoids, fatty acids, and phenolic compounds, including phenolic acids and flavonoids. One common method to stimulate plants for increased production of active compounds is subjecting them to various types of stresses, such as drought or salinity, along with plant growth regulators. This study investigated salicylic acid's effect on the phytochemical content and various biological properties of Roselle plants under water scarcity conditions.

The experiment was conducted as a factorial experiment in a completely randomized design, including irrigation intervals at three levels: three days (D1), five days (D2), and seven days (D3), and foliar application of salicylic acid at three levels: 0 (SA0), 0.75 mM (SA1), and 1.25 mM (SA2), with three replicates in a research greenhouse at the International Imam Khomeini University. The required seeds for cultivation were obtained from Pakanbazr Company in Isfahan. The imposition of drought stress through irrigation intervals of 3, 5, and 7 days started at the 4-leaf stage of the plants, and the plant spraying with salicylic acid was performed at the 6-leaf stage, three days before the start of irrigation treatments. The measurement of electrolyte leakage and malondialdehyde content was carried out to determine membrane stability and lipid peroxidation level. Additionally, the measurement of proline content and the activity of three antioxidant enzymes, including peroxidase, catalase, and ascorbate peroxidase, was performed in different samples. Furthermore, the total phenolic, flavonoid, and anthocyanin contents, as the main secondary metabolites of Roselle plants, were measured. The biological and medicinal properties, including the free radical scavenging activity (DPPH) and α-glucosidase inhibitory activity, were also evaluated. The obtained data were analyzed using SAS statistical software version 9.4. Mean comparisons were conducted using Duncan's multiple range test at a significance level of 5%.

The results showed both electrolyte leakage and malondialdehyde content increased significantly with increasing levels of applied drought stress on Roselle plants. However, overall, applying salicylic acid treatments led to a decrease in both indicators. The measurement of proline content in Roselle samples showed that the highest amount of proline was observed in plants subjected to the highest drought stress and treated with the 1.5 mM salicylic acid solution. The activity of all three antioxidant enzymes increased with the intensity of the applied drought stress and the concentration of salicylic acid solutions. Roselle's highest levels of phenolic and anthocyanin content were observed in samples sprayed with 1.5 mM salicylic acid solution. The change in anthocyanin content of Roselle plants also demonstrated that increasing the severity of water scarcity resulted in higher levels of this compound. The highest DPPH free radical scavenging activity was observed in Roselle samples grown under the highest drought severity and sprayed with the highest concentration of salicylic acid (61.93%). The highest antioxidant activity was also observed in samples treated with the highest concentration of salicylic acid solution in all irrigation regimes. The highest α-glucosidase inhibitory activity (69.2% and 67.7%) was associated with Roselle samples grown under the highest applied drought severity, sprayed with 1.5 mM and 0.75 mM salicylic acid solutions, respectively.

It can be concluded that the use of salicylic acid as a growth regulator can contribute to the enhancement of phytochemical content, especially secondary metabolites such as phenolic and flavonoid compounds, anthocyanins, and the elevation of defensive parameters such as the activation of antioxidant enzymes, as well as an increase in osmolyte content like proline in Roselle plants. These effects assist the plants in exhibiting greater resistance to drought stress and mitigating the negative impacts of this type of stress on the plant.