Effect of Seed Priming with Extracts of Beeswax Waste, Vermicompost, and Cordia Residues on Response of Marigold (Calendula officinalis) to Cadmium-induced Oxidative Stress

Extended Abstract Introduction: Cadmium, a heavy and hazardous metal with an atomic number of 48, is increasingly disseminated in agricultural soils and negatively affects plants due to its high solubility in water. The toxic distribution of this element in soils and plants leads to reduced germination, impaired plant growth, and diminished activities of antioxidant enzymes. Marigold, belonging to the Asteraceae family, possesses the ability to absorb cadmium from contaminated soils and is recognized as an ornamental and medicinal plant. Furthermore, employing seed priming techniques can enhance the plant's resistance to environmental stresses, including cadmium exposure. This study investigates the effects of marigold seed pre-treatment on growth, physiological traits, and antioxidant activity under cadmium stress conditions. Materials and Methods: To investigate the effects of marigold seed pre-treatment on growth and biochemical characteristics under cadmium stress conditions, a factorial experiment was conducted in a completely randomized design with three replicates at the Agricultural Higher Education Center in Bardseer, southest of Iran. The study included two factors: varying cadmium levels (0, 30, 40, and 60 mg/kg of soil) and four types of seed priming (beeswax pomace, vermicompost, cordia fruit extract, and no priming). To prepare organic extracts, dried and ground materials were mixed with distilled water and filtered after 48 h on a shaker. Seeds were soaked in these extracts for 12 hours before being sterilized and sown. The dry weight of aerial organs was measured, and biochemical characteristics such as phenolic content, anthocyanins, proline, protein, chlorophyll, carotenoids, and antioxidant enzyme activities were assessed. Analysis of variance was performed using SAS software, and mean comparisons were conducted using the LSD test. Results: The results indicated that increasing levels of cadmium stress significantly decreased the leaves/plant, flowers/plant, dry flower weight, dry plant weight, concentration of chlorophyll, protein, and soluble sugars, and catalase enzyme activity in marigold plants. However, cadmium stress elevated the content of the guaiacol peroxidase enzyme, anthocyanins, phenols, and proline concentrations. The effects of priming with different organic materials yielded varying results across different traits, with the effects of vermicompost extract and beeswax pomace being more pronounced compared to tamarind extract. Additionally, as the severity of cadmium stress increased, the positive role of seed priming in mitigating negative stress effects became more evident. Particularly, at the stress level of 60 mg of cadmium per kg of soil, seed priming using vermicompost and beeswax resulted in approximately 66% and 54% increases in mean plant dry weight, respectively, compared to the control, while at the stress level of 40 mg, the increases were 12% and 7%, respectively. However, under non-stress conditions and at the 60 mg per kg cadmium stress level, priming did not affect chlorophyll content. Seed priming with organic materials, especially vermicompost and beeswax pomace, exerted different influences on the activities of GPX and catalase antioxidant enzymes under both stress and non-stress conditions. Under non-stress treatment, priming did not affect guaiacol peroxidase enzyme activity; however, at cadmium stress levels of 30 and 60 mg per kg, seed priming with vermicompost yielded higher guaiacol peroxidase enzyme activity compared to the control and other treatments. At the stress level of 40 mg of cadmium per kg, no significant differences were observed among the seed priming with vermicompost, beeswax, and cordia extract compared to the control. Meanwhile, the application of all three priming extracts (vermicompost, beeswax, and cordia) significantly increased catalase enzyme activity across all levels of cadmium stress, with seed priming using vermicompost and, to a lesser extent, beeswax pomace producing higher catalase enzyme levels than the control and other treatments. In this study, soluble sugar content increased under all three cadmium stress levels with the application of priming materials, with vermicompost and, to a lesser extent, beeswax pomace showing the best effects among the priming treatments. Furthermore, beeswax and vermicompost extracts led to a 44% increase in phenolic content in marigold plants under 30 and 40 mg per kg cadmium stress conditions. Conclusion: The results of this study indicate that cadmium, as a heavy metal, has detrimental effects on the growth and biochemical attributes of marigold plants. According to the data obtained, increasing levels of cadmium resulted in a significant reduction in the leaves/plant, flowers/plant, dry flower weight, dry plant weight, chlorophyll and protein concentrations. However, cadmium stress led to an increase in the activity of antioxidant enzymes such as guaiacol peroxidase, as well as anthocyanins and phenols concentrations. Seed priming with organic materials, particularly vermicompost and beeswax pomace, significantly enhanced the plant's resistance to cadmium stress. At high levels of cadmium, these methods contributed to increased plant dry weight and enhanced antioxidant enzyme activities. Particulrlly, seed priming with vermicompost and beeswax pomace demonstrated greater efficacy in alleviating the negative effects of cadmium, allowing these methods to significantly maintain plant health under stress conditions through increased antioxidant activity and osmotic regulation. Therefore, these seed pre-treatment methods have the potential to be incorporated into agricultural optimization programs to enhance plant resistance to environmental stresses, particularly those arising from heavy metal contamination.