Exploring freeze-injury mechanism through ion-specific analysis of leachate from reversibly versus irreversibly injured spinach (Spinacia oleracea L.) leaves

The present study analyzed four cations (K⁺, Ca²⁺, Mg²⁺, Fe²⁺) in leachate from freeze-injured spinach (Spinacia oleracea L. ‘Reflect’) leaves exposed for four freezing-durations (FDs) (0.5, 3.0, 5.5, 10.5 h) at −4.8 °C. Comparison of electrolyte leakage from right-after-thaw with that after 6-d recovery revealed that injury at 0.5 or 3 h FDs was recoverable but irreversible at 5.5 or 10.5 h FDs. Data suggests leakage of K⁺, the most abundant cation in leachate, can serve as a proxy for total electrolyte-leakage in determining plant freezing-tolerance and an ionic marker discerning moderate vs. severe injury. Quantitative correspondence between Ca²⁺- and K⁺-leakage supports earlier proposition that leaked K⁺ induces loss of membrane-Ca²⁺, which, in turn, promotes further K⁺-leakage due to weakened membrane. Reduced/undetectable Fe²⁺ in leachate at longer FDs suggests activation of Fenton reaction converting soluble Fe²⁺ into insoluble Fe³⁺. Enhanced Mg²⁺-leakage at greater freeze-injury suggests structural/functional impairment of chlorophyll/chloroplast complex.