Post-harvest physiological deterioration of cassava (Manihot esculenta) - A review

Cassava (Manihot esculenta Crantz) is a valued root crop grown throughout the tropics for food, feed and biofuel and industrial uses. Fresh cassava roots have short shelf-life due to an endogenous phenomenon called Post-harvest Physiological Deterioration (PPD). The estimated losses fresh cassava roots due to PPD is nearly 1/3rd of total harvest world-wide. PPD symptom starts within 24 hr after harvest, initially as blue black discolouration in the storage parenchyma which quickly spread to entire root. The roots become unfit for consumption within 2-3 days after the harvest in most cases. Several lines of evidence suggest that the PPD of cassava roots is an enzymatic, endogenous oxidative process. Reactive Oxygen Species (ROS) have been shown to increase very early during PPD and strongly correlated with the symptoms and also evidenced by the active involvement of key ROS enzymes during PPD. Increase of phenolic compounds in freshly harvested roots (e.g. scopoletin), accumulation of H2O2, and other wound induced changes and inherently low protein content of roots tissue are also implicated for the rapid deterioration. Conventional PPD managements like waxing of fresh cassava roots and polybag storage with suitable fungicides are normally recommended and offer relief to certain extent. Efforts to breed delayed PPD in cassava had met with little success. Several attempts to manipulate the ROS metabolism through recombinant technologies proved little success at field level. Understanding the wound response of cassava roots and its signalling network is essential to evolve strategies to delay the PPD and prolong the shelf-life of fresh cassava roots.