Mono‐carboxylic acids and their salts inhibit wound‐induced phenolic accumulation in excised lettuce (Lactuca sativa) leaf tissue

Abstract

Wounding, as during excision and preparation of lettuce (Lactuca sativaL.) leaf tissue for salads, induces the synthesis and accumulation of phenolic compounds that participate in subsequent reactions that cause tissue browning. Exposure of excised 5‐mm mid‐rib segments of romaine lettuce leaf tissue to vapors of mono‐carboxylic acids or aqueous solutions of mono‐carboxylic acids or their salts inhibited wound‐induced phenolic accumulation (WIPA) and subsequent tissue browning. The decline in phenolic content followed a quadratic curve with increasing concentration, reaching a maximum inhibition after 60 min of 74 ± 8% for 50 mMsodium acetate (2 carbons, C2) and 91 ± 4% for 20 mMsodium decanoate (capric acid, C10). Respiration (i.e. carbon dioxide production) was unaffected by concentrations of formic, acetic, or propionic acids that reduced wound‐induced phenolic content or that increase ion leakage from the tissue into an isotonic mannitol solution. However, WIPA was suppressed up to 70% at concentrations (20 mMacetate) that did not increase ion leakage over that of water controls. Various acetate salts (i.e. ammonium, calcium, magnesium, and sodium) all produced the same level of inhibition. The effectiveness of the compounds increased with increasing number of carbons in the molecule from 1 to 10, and was unaffected by whether the carbons were a straight chain or branched or whether the treatment was delayed by up to 6 h. The effectiveness of butyrate (C4) in reducing WIPA (27% reduction at 20 mM) was less than that predicted from the response of the two adjacent mono‐carboxylates similarly applied: propionate (C3) (62%) and valerate (C5) (73%). It appears that, unlike the n‐alcohols, mono‐carboxylates are not interfering with the synthesis or propagation of a wound signal but are interfering with subsequent steps in the production and accumulation of wound‐induced phenolic compounds.