عنوان مقاله [English]
Water deficit is one of the most fundamental problems of agricultural systems throughout the world, which influences both yield and quality of the products. Applying compounds such as salicylic acid (SA) can improve plant resistance against environmental stresses. The present study was conducted to evaluate the impact of drought stress and SA application on quantitative and qualitative traits of holy basil (Ocimum sanctum), based on a factorial experiment in the form of a completely randomized design in three replications. The experimental treatments included drought stress at three levels (100, 70, and 50% FC) and foliar application of salicylic acid at three levels (0, 0.75, and 1.5 mM). Obtained results showed that water deficit caused a decrease in plant growth factors, RWC, and the amount of photosynthetic pigments, while foliar application of salicylic acid increased the above-mentioned parameters. Drought stress and foliar application of salicylic acid increased proline content, total content of phenolic compounds, flavonoid compounds and DPPH free radicals inhibition. In addition, the percentage of essential oils increased under drought stress. Also, plants that were sprayed with salicylic acid, showed a significant increase in essential oil percentage compared to the control. By reducing irrigation water, the percentage of eugenol in essential oil was decreased, but foliar spraying with a high concentration of salicylic acid increased the percentage of this compound. The experimental treatments also had a significant effect on the percentage of 1, 8-cineole and estragole compounds. Therefore, the application of a proper amount of SA can be considered a practical and simple strategy for increasing the quality and quantity of holy basil in arid and semi-arid areas.
Ocimum sanctum, a member of the Lamiaceae family, is a vital medicinal and aromatic plant with a global presence. Adequate irrigation is pivotal for optimal growth and development of Holy Basil, as well as other Ocimum species. Drought, a major abiotic stressor, significantly impacts plant health and agricultural productivity worldwide. Drought stress, characterized by inadequate water availability including precipitation and soil moisture throughout a plant's life cycle, critically affects physiological and biochemical processes, including photosynthesis, respiration, ion uptake, and nutrient metabolism. To mitigate the detrimental effects of drought stress, plants employ various mechanisms known as avoidance, escape, and tolerance. Salicylic acid (SA), classified as a phenolic growth regulator, plays a crucial role in regulating plant growth and development. It serves as a potent signaling molecule in plant defense by modulating physiological and biochemical functions.
Material and Methods
This study utilized a factorial design with three replications in a randomized configuration within a research greenhouse at the International University of Imam Khomeini. Experimental treatments included three levels of drought stress (W1: 100%, W2: 70%, W3: 50% of field capacity) and three levels of foliar salicylic acid spray (S0: 0 mM, S1: 0.75 mM, S2: 1.5 mM). Seeds obtained from Bazran Company were sown at 1.5 cm depth in each pot, with five seedlings per pot. SA foliar spray occurred at the 6-leaf stage, three days prior to drought stress treatments. Growth parameters, such as plant height, fresh and dry weight, and number of branches, were assessed at full flowering. Standard methods measured relative water content (RWC), photosynthetic pigments, proline content, total phenolic and flavonoid compounds, DPPH free radical inhibition, essential oil percentage, and composition.
Results showed evident negative impacts of water deficit on plant growth factors, RWC, and photosynthetic pigment content. In contrast, foliar salicylic acid application improved these parameters. Drought stress, combined with salicylic acid treatment, increased proline content, total phenolic and flavonoid compounds, and DPPH free radical inhibition. As drought intensity and salicylic acid concentration increased, holy basil plants exhibited significantly elevated total phenolic content. For instance, the S2W3 treatment recorded the highest phenolic compound level (65 mg GAEs/g DW), whereas the lowest was in S0W1 (30.27 mg GAEs/g DW). A similar pattern was observed for flavonoid content, with S2W3 at 18.75 mg QEs/g DW, and S0W1 and S0W2 at 12.41 and 12.98 mg QEs/g DW, respectively. The S2W3 treatment demonstrated the highest DPPH scavenging activity (57.90%), and S0W1 the lowest (31.08%). Essential oil percentage increased under drought stress and also application of salicylic acid. Eugenol percentage decreased with drought but increased with high salicylic acid concentrations. The highest eugenol percentage was from the W1 treatment (21.29%), and salicylic acid application (S2) at 20.20% compared to control. Notably, the experimental treatments significantly affected the percentage of 1 and 8-cineole and estragole compounds. Drought stress conditions (W2 and W3) led to the highest levels of 1 and 8-cineole (19.82% and 20.55%, respectively). Additionally, a higher salicylic acid concentration (S2) substantially increased (20.74%) the amount of this compound in holy basil essence. Estragole levels were highest in S2W1 and S2W2 (13.4% and 13.76%), and lowest in S0W1 (10.06%).
Results indicated that reduced irrigation slows Holy Basil growth due to impaired cell division, elongation, and differentiation, controlled by water availability. Growth decline might result from reduced chlorophyll content, photosynthesis, and cell division due to drought. The impact of salicylic acid on growth could be linked to hormonal changes, improvements in photosynthesis, transpiration, and stomatal conductance. However, the effect of the external application of salicylic acid on plant growth depends on species, growth stage, and concentration. Salicylic acid regulates growth, cell division, and physiological processes, enhancing plant adaptability.
Adverse impact of water scarcity on yield quality and quantity of holy basil can be mitigated through salicylic acid application, protecting against drought. Salicylic acid significantly improves physiological processes, including photosynthetic pigment content and water retention. The role of salicylic acid is not limited to immediate plant defense; it exerts long-term effects on growth and metabolism, preparing plants for future stress. Thus, foliar application of salicylic acid at 1.5 mM can be an economical strategy to enhance holy basil yield under drought conditions.