Cell wall modifications and ethylene-induced tolerance to non-chilling peel pitting in citrus fruit

Non-chilling peel pitting (NCPP), a storage disorder resulting in the formation of depressed areas in the peel of many citrus cultivars, is reduced by ethylene treatments. We hypothesized that this effect may be associated with biochemical changes of cell wall components. Therefore, we extracted cell wall material from albedo and flavedo tissues of 'Navelate' oranges stored in air, conditioned with ethylene (2 μL L−1) for 4 days and subsequently transferred to air, or continuously stored in an ethylene-enriched atmosphere (2 μL L−1). Uronic acids and neutral sugars were extracted into five fractions enriched in specific wall polymers namely water-, CDTA-, Na2CO3-, and 1 and 4 M KOH-soluble fractions. Pectin insolubilization was found in control fruit at long storage times. Ethylene treatments, alleviating NCPP, increased polyuronide solubility in the albedo and had a slight effect on the flavedo. Ethylene-treated fruit showed greater content of water-soluble neutral sugars and a larger proportion of hemicelluloses readily extractable with 1 M KOH, with a concomitant reduction in the 4 M KOH-soluble fraction. This suggests that the protective role of ethylene on NCPP is associated with an increased solubilization of the wall of albedo cells.