Salinity Stress in Calendula officinalis: Negative Growth Impacts Offset by Increased Flowering Yield and the Mitigating Role of Zinc

Salinity stress is a significant abiotic factor that limits plant growth and productivity by causing ionic imbalances and oxidative damage. Chelated zinc (Zn) has gained attention as an effective micronutrient to mitigate salinity-induced stress by enhancing antioxidant defense mechanisms, osmotic regulation, and physiological processes. This study aimed to investigate the impact of foliar-sprayed chelated Zn on the alleviation of salinity stress in Calendula officinalis. A pot experiment was conducted with varying salinity levels (0, 1000, 2000, and 3000 ppm NaCl) and Zn concentrations (0, 200, 400, and 600 ppm). The results demonstrated that chelated Zn significantly enhanced the growth parameters, flower yield, and biochemical traits, particularly under high-salinity conditions. Salinity stress was associated with a marked increase in the Na⁺ and K⁺ concentrations and a reduction in the Zn levels in the leaves. However, the foliar application of chelated Zn reduced the Na⁺ and increased the K⁺ concentrations in the leaves, resulting in an elevated K⁺/Na⁺ ratio with higher salinity and Zn application rates. Furthermore, the salinity and chelated Zn treatments stimulated the production of proline, phenols, flavonoids, and antioxidant activity, indicating the plant’s adaptive mechanism to enhance its secondary metabolite production under stress. These findings highlight the potential of chelated Zn to improve the salinity tolerance, supporting sustainable agricultural practices in saline-affected areas. Although salinity reduced the overall growth of C. officinalis, farmers are encouraged to cultivate this plant for its valuable inflorescences under saline irrigation conditions (up to 2000 ppm), combined with chelated Zn foliar applications at 400–600 ppm. We also recommend further research on other micronutrients.