PPD- Post-harvest Physiological Deterioration

Upon harvesting, the starchy roots of cassava suffer a rapid deterioration that renders them unpalatable within 24–72 hours.
This deterioration is a purely physiological process, though microbial deterioration can set in subsequently. Physiological deterioration shows up as discoloured vascular tissues and storage parenchyma, these changes are typical of a plant's response to wounding. Strong florescence under Ultraviolet light is due to the production of phenolic compounds including scopoletin and tylose. PPD results in increases in respiration and mobilisation of starch to sugars by the activity of acid invertase. Ethylene biosynthesis also increases, and is considered to play a role in coordinating wound responses. Changes in lipid composition and increased synthesis of phenols, diterpenes and catechins also occurs. Enzyme activity increases too e.g. Phenylalanine Ammonia Lyase-PAL, Catalase-CAT, Polyphenol Oxidase-PPO, invertase and peroxidase and changes in gene expression & protein synthesis. PPD seems to be partly the result of oxidative processes.
PPD is not degenerative; rather it is an active response of the cassava root involving changes in gene expression, protein synthesis and secondary metabolite accumulation. This is strong evidence for the presence of genes whose activities could be modified in order to effect alterations in the PPD response. There is a genetic contribution to PPD. However, this is strongly influenced by the environment.

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