| Fruit softening is a significant event during ripening and has been widely accepted to be the result of the alteration of cell wall components mediated by cell wall hydrolases. In papaya, cell wall-hemicelluloses are extensively modified as fruit start to soften. Endoxylanase has been found to be a prominent hemicellulose modifying-enzyme during papaya fruit ripening and may play a role in the softening process. Studies on three papaya cultivars: 'Line 8', 'Sunset', and 'Line 4--16' showed differences in softening behavior. Each cultivar showed a unique relationship between softening, skin color changes, ethylene production and respiration rate. Endoxylanase activity was detected in all papaya cultivars. The relationship study showed that when papaya fruit started to soften, endoxylanase gene expression was highly related with endoxylanase protein accumulation and activity. This relationship pattern was continued to the end of ripening and was found consistent in all papaya cultivars tested. The effect of MCP (1-methylcyclopropene) as an ethylene action inhibitor for delaying papaya fruit ripening was also investigated in this work. Papaya fruit following MCP treatment showed a delay in climacteric respiration, ethylene production, skin color development and softening. MCP had more effect in delaying papaya fruit ripening at earlier stage of ripening (less than 30% skin yellowing stage). Fruit after treatment with MCP at less than 10% skin yellowing stage showed a significant delay in softening and caused the rubbery texture when ripened. Papaya fruit that had the rubbery texture showed markedly suppressed in endoxylanase gene, protein and enzymatic activity. Endoxylanase gene expression, protein accumulation and activity might be interrupted and an incomplete recovery was in papaya fruit following MCP treatment. The effect of MCP on the specific cell wall enzymes such as endoxylanase, indicated ethylene regulation contributed toward fruit softening. |
