The Development of Occlusions in Cassava (Manihot esculenta Crantz) Root Xylem Vessels

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Abstract
Cassava roots exposed to physiological stress arising from wounding, respond by forming coloured occlusions in the xylem vessels and producing chemically similar deposits in the storage parenchyma. Colourless occlusions consisting of lipids, carbohydrates and phenols, including leucoanthocyanidins and catechins were present in a limited number of xylem vessels in freshly harvested cassava roots. These occlusions were found to increase in frequency in injured roots especially when stored at low humidity. Concurrent with the appearance of coloured products in the xylem vessels there was a loss of response to tests for phenols, but an increase in lignin-like material. However, the presence of lignin in these occlusions was not substantiated: the lignin-like responses were probably produced by condensed tannins with lignin-like properties formed by the polymerization/condensation of catechins and leucoanthocyanidins. Observed increases in polyphenol oxidase and peroxidase activities could be related to such reactions.

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