Combined Supplementation with Iron, Manganese and Zinc Sulfates Improves Growth and Modulates Leaf Gene Expression in Salt-Stressed Rhizophora stylosa Seedlings

Rhizophora stylosa seedlings were grown at three initial NaCl levels (0, 100 and 400 mM), with individual or combined iron, manganese and zinc sulfate supplementation at the two saline levels. Eleven treatments comprised three independent pots each. Growth, root architecture, photosynthesis, tissue elements and biochemical responses were assessed after 90 days. Relative to SCK, the high-salt control receiving basal nutrients without supplemental micronutrients, combined Fe + Mn + Zn (SFeMnZn) increased the dry mass by 29.8%, net photosynthetic rate by 67.0% and total root length by 133.1% (all adjusted p < 0.01). Maximum photosystem II photochemical efficiency increased from 0.659 to 0.795 (adjusted p < 0.001). Zn produced the strongest overall growth response among individual supplements, although their dry-mass differences were not significant. Combined supplementation increased tissue micronutrient concentrations, reduced oxidative injury and altered antioxidant and osmolyte responses. Leaf RNA sequencing of SFeMnZn versus SCK identified 1024 differentially expressed transcript entries associated with photosynthetic, redox, carbon-metabolic and cation-exchange processes. The findings support evaluation of the tested sulfate supplementation regime for controlled nursery cultivation and identify molecular candidates for further investigation.