Effects of salicylic acid on peanut (arachis hypogaea l.) plant under drought stress

In this thesis study, Halisbey peanut (Arachis hypogaea L.) plants were subjected to drought stress induced by polyethylene glycol (PEG-6000). The study aimed to determine the degree of damage caused by drought stress and to evaluate whether this damage could be mitigated by salicylic acid (SA) at different concentrations (1 mM and 10 mM). For this purpose, the photosynthetic pigment contents (chlorophyll-a, chlorophyll-b, total carotenoid), proline content, MDA content, total phenolic and total flavonoid contents, and fatty acid contents in control and all treatment plants were evaluated. Accordingly, plant seeds were sown in pre-prepared pots (peat:soil:perlite) and allowed to grow in a plant growth chamber with controlled conditions. The plants were irrigated with 1/4 Hoagland nutrient solution for 5 weeks, and after this period, different ratios of PEG (%3 and %9) and PEG+SA applications were applied for 2 weeks, except for the control group (0 mM PEG). After a total growth period of 5 weeks, PEG and SA applications were applied to the plants for 2 weeks, and the plants were harvested after 14 days.To determine the changes in photosynthetic pigment contents caused by drought (PEG) stress in Halisbey peanut plants, the effects on chlorophyll-a, chlorophyll-b, and total carotenoid contents were evaluated comparatively. The results for chlorophyll-a content showed that decreases observed with PEG applications were reversed by the addition of SA. The highest increases in these applications were obtained from plants in the %3 PEG+10 mM SA and %9 PEG+10 mM SA groups, with 1.442 µg/g FW and 1.408 µg/g FW, respectively. Both %3 PEG and %9 PEG applications combined with SA also led to an increase in chlorophyll-b content. The increases in chlorophyll-b content with 10 mM SA addition were higher compared to the groups with 1 mM SA. Similarly, the total carotenoid amounts in plants subjected to %3 and %9 PEG stress decreased compared to the control, while the separate application of SA caused an increase in content.To evaluate the effects of damage on membranes, changes in MDA contents in peanut plants were examined, and the MDA content in the %3 PEG application increased approximately 2 times compared to the control group, while it increased approximately 3 times in the %9 PEG application. The addition of SA to the environment resulted in approximately a 50% reduction in MDA contents. The MDA content, which was 5.548 µmol/g FW in the %3 PEG application, decreased to 2.939 µmol/g FW (%3 PEG + 10 mM SA) and from 6.339 µmol/g FW in the %9 PEG application to 3.070 µmol/g FW in the %9 PEG + 10 mM SA group. When all treatment groups were evaluated in terms of total phenolic/flavonoid contents, higher ratios were obtained from extracts with 10 mM SA addition. The highest value for total phenolic content was obtained from the %3 PEG + 10 mM SA application, with 87.429 μg GAEs/mg extract, followed by the %9 PEG + 10 mM SA application with 67.480 μg GAEs/mg. The %3 PEG + 10 mM SA application resulted in a higher total phenolic content compared to the %9 PEG + 10 mM SA application. The %3 PEG might have enhanced the plant's capacity to cope with stress more efficiently, leading to increased synthesis of phenolic compounds. This indicates that %3 PEG has a more optimal effect on the plant.For total flavonoid content, the highest value was obtained from the %3 PEG application group, with 47.279 μg QEs/mg extract, followed by the %9 PEG + 10 mM SA application with 27.018 μg QEs/mg. The reason for the highest flavonoid content with %3 PEG might be that it creates an environment where stress responses in plants are most efficiently triggered, and flavonoid production is most stimulated at this level. The %9 PEG caused a slightly limited increase, indicating that while PEG's capacity to create osmotic stress is significant, too much stress can adversely affect the plant.In conclusion, PEG has a strong enhancing effect on flavonoid production, but this effect may be limited as PEG concentration increases. Additionally, SA can support this process but may not be as effective as PEG alone.In terms of proline content, the highest value was observed in the %9 PEG + 10 mM SA application (12.779 mmol/g FW), with a 4-fold increase compared to the control and a 3-fold increase compared to the %9 PEG application. In this thesis study, fatty acid analyses were also carried out using hexane extracts prepared from peanut plants. When the results were evaluated, the fatty acids that showed an increase compared to the control group in the %3 PEG application were palmitic acid, linoleic acid methyl ester, phytol, and heptacosane. In the %9 PEG application, the fatty acids that showed an increase were heptadecan, palmitic acid, linoleic acid methyl ester, phytol, heptacosane, and squalene. The major fatty acids in all application groups were palmitic acid, oleic acid methyl ester, and linoleic acid methyl ester, while the minor fatty acids were heptacosane and squalene.These study findings indicate that SA applications, particularly at a 10 mM SA concentration, have the potential to mitigate the adverse effects of drought on Halisbey peanut plants under drought stress conditions. However, it should be noted that the effects of SA may vary depending on the type of stress, the degree of stress, conditions, application method, and plant species. The limited number of studies in this field further underscores the originality and contribution of this research to the literature. Therefore, much more research is needed to fully understand the effects of phytohormones like SA in stress management strategies. The data obtained from this thesis and possible future studies can help us better understand the effectiveness of SA and similar phytohormones against stress and uncover the resistance and adaptation strategies of plants under various environmental stresses (drought, salinity, etc.).