Heat shock increases mitochondrial H2O2 production and extends postharvest life of spinach leaves

The effects of moderate heat shock (HS) treatments on the senescence of detached spinach leaves have been studied. At harvest, detached spinach leaves received moderate heat treatments (37, 40, 43 °C and room temperature) by immersion in water baths. The content of small HS proteins increased proportionally to the temperature applied, with undetectable levels in untreated leaves. A HS treatment at 40 °C delayed leaf senescence as indicated by higher chlorophyll content and potential quantum yield of PSII (Fv/Fm), and decreased solute leakage after storage for 7 d compared with untreated samples. A higher in vivo production of H2O2 was observed as HS temperature increased. Oxidation of 2′,7′-dichlorodihydrofluorescein diacetate observed by in vivo confocal microscopy revealed that mitochondria were the main site of reactive oxygen species generation under either untreated or HS-treated leaves. Although high-temperature treatments did not prevent the loss of water soluble antioxidant concentrations, the ratio of reduced/oxidised forms of ascorbic acid was higher 3 d after HS treatment, compared with control leaves. The effect of HS treatments on physiologically based protective mechanisms that delay leaf senescence is discussed.