Abstract
<title>Abstract</title> <p>Understanding how plants spatially manage salt ions is critical for explaining genotypic differences in salt tolerance. We compared two maize (Zea mays) genotypes with contrasting tissue tolerance, namely ES-Metronom (tolerant) and LG30222 (sensitive) to examine how epidermal ion partitioning influences stomatal regulation. Salt treatment (50 mM NaCl) increased leaf Na⁺ and Cl⁻ concentrations without impairing photosynthesis. Despite higher Na⁺ accumulation, the tolerant ES-Metronom maintained superior gas exchange, indicating a robust tissue tolerance strategy. Cryo-scanning electron microscopy with energy-dispersive X-ray spectroscopy (Cryo-SEM-EDX) provided a semi-quantitative analysis of cell wall relative ion enrichment patterns, revealing that signals were confined to the outer 2–3 µm of the leaf epidermis. Under salinity, ES-Metronom exhibited a distinct "cell wall sink" strategy, preferentially retaining Na⁺ and Cl⁻ in the walls of subsidiary and pavement cells while limiting enrichment in guard cells. This cell-type specificity was absent in the sensitive LG30222. Principal component analysis (PCA) confirmed that this coordinated epidermal partitioning strongly correlates with sustained gas exchange. Together, these findings reveal an underappreciated mechanism of ion homeostasis, whereby selective, cell type–dependent ion retention in cell walls supports photosynthetic resilience under salinity.</p>