Introduction: Henna, or Lawsonia inermis L., is a plant native to Southwest Asia and North Africa from the Lythraceae family that possesses many biological properties, including antifungal, antibacterial, and antioxidant activities. Henna is a valuable medicinal and ornamental plant with various applications in the cosmetic, pharmaceutical, and dyeing industries. Drought stress is one of the most significant factors limiting the growth and survival of plants in various regions of Iran, as it affects the physiological and biochemical activities of plants by reducing soil water capacity. The phytohormone salicylic acid, which is based on phenols, regulates several essential physiological functions of plants, including transpiration, photosynthesis, ion transport, and water uptake. Sodium nitroprusside is a nitric oxide-releasing chemical. Nitric oxide is a small gaseous signaling molecule in plants that is crucial for growth and development under both biotic and abiotic stress conditions, as well as in natural physiological settings.
Materials and Methods: This experiment was conducted in Qaleh Ganj city, Kerman province, using a split-plot design based on a randomized complete block design with three replications in 2021. The main factor included three levels of drought stress (50%, 75%, and 95% of field capacity), while the sub-factor involved foliar spraying (control, salicylic acid, sodium nitroprusside, and a combination of salicylic acid and sodium nitroprusside). Seeds of the henna plant (Roodbar ecotype) were obtained from the Faculty of Agriculture at Bahonar University. Table 1 presents the soil characteristics of the experimental site. Planting was carried out directly in the main field, with a density of 20 plants per square meter. The growth regulator solution was sprayed at the five-leaf stage of the plants. Drought stress treatment continued until harvest, and the foliar treatment was applied only at the five-leaf stage. After removing the marginal effects, the existing plants were harvested, and the leaf yield was determined. The planting date for the henna plant was April 1, and the harvesting date was November 5.
Results and Discussion: Drought stress reduced the number of leaves per plant, as well as biological and leaf yields, while increasing the activity of polyphenol oxidase and superoxide dismutase, along with the levels of total phenolic compounds and anthocyanins in henna plants. The combined application of salicylic acid and sodium nitroprusside significantly affected most of the traits studied. The application of this combination increased the number of leaves per plant by 10.07%, the activity of the polyphenol oxidase enzyme by 22.22%, the total phenolic content by 7.18%, the biological yield by 8.54%, and the leaf yield by 21.97%. Using the combination of salicylic acid and sodium nitroprusside enhanced the antioxidant defense system of the henna plant.
Conclusion: In general, the results of this study indicated that drought stress negatively affected the metabolism of the henna plant, resulting in a decrease in the number of leaves per plant, as well as biological and leaf yields. Drought stress also increased the activity of the enzymes polyphenol oxidase and superoxide dismutase, along with the levels of total phenolic compounds and anthocyanins. However, the application of a combination of salicylic acid and sodium nitroprusside increased the number of leaves per plant, enhanced antioxidant enzyme activity, and improved phenolic content, biological yield, and leaf yield.