Anatomical adaptation of water-stressed Eugenia uniflora using green synthesized silver nanoparticles and melatonin

Cell and sub-cellular anatomical adjustments are adaptations utilized by plants to tolerate abiotic stress. Both melatonin and Morinda lucida-silver nanoparticles (ML-AgNPs) are recognized as bio-stimulants. The study examined the morphological changes and adaptive characteristics of these bio-stimulants under water-stress Eugenia uniflora. Twenty-four hours was spent priming the seeds with melatonin (0.06 mg/L), ML-AgNPs (0.06 mg/L), and a mixture (1:1) of the two. The seeds were sown and subjected to water stress for 7 days. The leaves, stems, and roots of water-stressed E. uniflora were sectioned, dried, and examined using a microscope. Drought stress led to the production of non-glandular trichomes on the abaxial and the transformation of paracytic stomata into diacytic stomata. During water stress, melatonin enlarges intercellular gaps and stomata, increases sponge and palisade parenchyma, and thickens epidermis (stem and root) and fibers. The ML-AgNPs diminished the size of mesophyll, intercellular gaps, stomata, and stem fiber. The ML-AgNPs increased the size of bulliform cells and activated the mechanical resistance features of sclerophyllous leaves (thick-celled epidermis and sclerieds) and ray parenchyma (root and stem). Equally, Melatonin and ML-AgNPs increased stem and root anatomical characteristics (xylem, bark, pith, cortex, epidermis, and vascular bundles). Stomata of E. uniflora are susceptible to alterations and undergo cell division into two new stomata (stomatogensis) in response to varying conditions (melatonin and ML-AgNPs). Melatonin adopted a strategy for maintaining a high plant water status, possibly by osmoregulation, whereas E. uniflora primed with ML-AgNPs survived by minimizing transpirational water loss through morphological changes.