Cell membrane dysfunction in postharvest spinach leaves increases water permeability through parallel pathways involving the lipid bilayer and aquaporins

Leafy greens, such as spinach, are susceptible to postharvest deterioration. Understanding factors underlying freshness loss, particularly related to cell membrane function and water permeability, is crucial for improving postharvest handling and preservation techniques. This study aimed to investigate changes in membrane water permeability in spinach mesophyll cells during storage and to identify the factors that affect these changes. We examined three properties of the cell membrane: lipid peroxidation, which triggers cell membrane deterioration; membrane rupture strength, reflecting the viscosity and fluidity of the cell membrane; and the expression of four aquaporin (AQP) genes: SoPIP1;1, SoPIP1;2, SoPIP2;1, and So delta TIP. To analyze their relationship with membrane water permeability, we conducted a protoplast shrinking assay under a hyperosmotic challenge. Our findings revealed that lipid peroxidation increased and membrane rupture strength weakened in response to varying oxygen concentrations during storage, correlating with an increase in membrane water permeability. Interest-ingly, the expression of SoAQPs markedly decreased during storage, regardless of the oxygen concentration. These observations suggest that membrane peroxidation and embrittlement promote packing imperfections in the lipid bilayer and induce AQPs to adopt an open conformational state, thereby contributing to the overall increase in membrane water permeability, even when the expression of SoAQPs decreases.