Guard Cell Chloroplast Remodeling Under Salt Stress Correlates with Ion Enrichment in Faba Bean

Salinity stress alters cellular ion homeostasis and photosynthetic activity, yet how guard cell chloroplast architecture contributes to salt adaptation remains poorly understood. Here, we investigated salt-induced chloroplast remodeling in guard cells of two faba bean genotypes, Fuego and Scoop, by integrating 3D chloroplast imaging, ion enrichment analysis and photosynthetic measurements. Salt stress induced distinct genotype-dependent changes in chloroplast morphology, with Fuego exhibiting pronounced chloroplast enlargement and reduced surface area-to-volume ratios under Na₂SO₄ and high NaCl, whereas Scoop showed treatment-dependent remodeling with larger chloroplasts under low NaCl and higher surface area-to-volume ratios under Na₂SO₄ and CaCl₂. These structural responses were associated with differential Na and Cl partitioning at the stomatal complex surface. In Fuego, chloroplast size was negatively associated with photosynthetic rate, whereas Scoop showed positive relationships between chloroplast size and photosynthetic performance. Multivariate analysis further revealed coordinated associations among chloroplast architecture, ion enrichment and photosynthesis that distinguished the two genotypes under salinity. Our findings demonstrate that guard cell chloroplast remodeling is closely associated with genotype-specific salt responses and local ion partitioning. Integrating organelle structural plasticity with local ion homeostasis provides a spatially resolved perspective on the cellular basis of genotype-dependent salinity adaptation.