Ultrastructural, Secondary Metabolite, and Antioxidant Modulation in Response to Salt-Affected Habitats Induced Oxidative Stress and Their Accumulation in Malva parviflora L. and Rumex dentatus L.

Salinity stress is a major abiotic factor that affects medicinal plant growth, performance, and secondary compounds. Malva parviflora L. and Rumex dentatus L. plants were collected from three habitats in the northeastern Nile Delta governorates of Damietta and El-Dakahlia. Conductivity (salinity) classified the habitats as mesophytic, moderately saline, and saline. Chemical and physical soil characteristics varied by habitat. Results show that M. parviflora L. and R. dentatus L. had high soluble sugars, total carbohydrates, electrolyte leakage, and proline in the saline habitat. In contrast, mesophytic habitats showed low content. In addition, R. dentatus L. had more antioxidant enzymes and elements in saline habitats than in mesophytic habitats. In saline habitats, M. parviflora L. and R. dentatus L. were characterized by more calcium and sodium increase than mesophytic habitats. Moreover, R. dentatus L. had more phenols, alkaloids, flavonoids, anthocyanin, and tannins under saline conditions than M. parviflora. Meanwhile, in the saline habitat, plant hormones, i.e., indole acetic acid and gibberellic acid, decreased significantly in both M. parviflora and R. dentatus than in the mesophytic habitat. Scanning Electron Microscopy (SEM) of the tested plants showed the highest stomatal frequency and area on the lower surface of mesophytic plant leaves compared to either its upper surface or both leaf surfaces in saline habitat. Hence, it can be concluded that R. dentatus plant can mitigate the negative effects of salinity by improving the qualitative and quantitative performance under salinity stress more than M. parviflora plant.