Visualization of the biochemical dynamics of tannins, proteins, starch, and pectin in the endosperm and embryo of maize (Zea mays L.) during germination under salt stress and non-stress conditions

Corn is one of the most economically important cereal crops cultivated around the world, and understanding the proper mobilization and utilization of starches, sugars, pectin, and metabolites in seeds is crucial for enhancing effective germination and promoting the development of healthy seedlings. Salt stress hinders seed germination by limiting water uptake, damaging cell structures, disrupting key metabolic processes, and impairing the function of endosperm and embryo tissues. However, visual representations and imaging of biochemical changes in the tissues and cells of the embryo and endosperm within corn seeds during germination under salt stress have been limited in the literature. In this study, we aimed to compare the structural changes and visualize the biochemical alterations occurring within the seed tissues (i.e., the outermost layer of the corn seed, endosperm, and embryo) at 6, 24, 48, and 96 h during germination under non-stress control conditions (0 dS m⁻¹) and salt stress conditions (4 dS m⁻¹ and 8 dS m⁻¹) using light fluorescence and scanning electron microscopy (SEM). We observed that pores between the pericarp cells (epidermis), the outermost part of the seed, did not shrink or deform under salt stress. In contrast, under non-salt stress conditions, the intercellular spaces between the pericarp cells opened rapidly. This limited porosity in the outermost layer cells could lead to inadequate water absorption, significantly contributing to delayed or failed germination under salt stress. Starch, the primary storage carbohydrate found in the endosperm, decreased rapidly under non-salt stress conditions; however, starch degradation slowed under salt stress. Almost no starch grains were detected in the embryo or scutellum cells. Protein depletion in aleurone cells occurred rapidly during germination in the absence of salt stress but slowed as salt stress increased. We also observed that the movement of metabolites from the embryo to the root and shoot regions within the seed was slowed or diminished under saline conditions. Alterations in cellulose and tannins were observed during seed germination under both salt and non-salt stress conditions; however, the scutellum cell walls retained their structural integrity due to partial, localized degradation. During germination, endosperm cell walls became more distinct, with an increased presence of pectin and mucopolysaccharides. The shoot region, initially rich in proteins, sugars, tannins, and pectins, was also found in the mesocotyl cells and intercellular spaces. Visualizing these changes in seed structure throughout our study enhances our understanding of the adverse conditions.