Comparative Physiochemical Mechanisms of Salt Tolerance between Cornus florida and Cornus hongkongensis subsp. elegans Based on Germination and Seedling Growth

Cornus florida and Cornus hongkongensis subsp. elegans are potential species in saline-alkali areas for use as horticultural plants with high ornamental value. This work aimed to screen suitable tolerant dogwoods by comparing the responses of two dogwoods to salt stress according to phenotypic and physiochemical indicators on germination and seedling growth. For relieving seed dormancy, two pretreatments were performed on seeds, coded as T1 (soaked by gibberellic acid (GA3) alone) and T2 (soaked by GA3 with gradient salt concentrations of 0.2%, 0.3%, 0.4%, and 0.45%), and pretreated seeds were then germinated at the corresponding salt concentrations in pretreatment. As salt levels increased, the seed germination parameters decreased for both dogwoods, whereas the relative salt damage rate (SDR) significantly increased. While the growth of above-ground parts was significantly inhibited, that of underground parts changed little. Furthermore, the malondialdehyde content (MDA), superoxide dismutase activity (SOD), organic solutes (soluble sugar (SS) and proline (Pro)), and inorganic ions (K+, Na+, Ca2+) significantly accumulated in seedlings with increasing salinity levels, whereas K+/Na+ ratio decreased. Correlation analysis indicates that organic osmolyte (SS) and ion osmoregulation could be the main physiological mechanisms conferring salinity tolerance for both dogwoods. Compared with osmolytes, ion osmoregulation may be more critical for salt tolerance in the seedlings of the studied dogwoods. Collectively, the superior salt tolerance of C. florida to C. hongkongensis subsp. elegans suggests that the two species could be cultivated in saline areas with 0.3% and 0.2% salt levels, respectively.