Colour of blackspot bruises in potato tubers during growth and storage compared to their discolouration potential

Abstract Two potato (Solanum tuberosum) cultivars, Dali and Oleva, with, respectively, low and high susceptibility to blackspot bruise, were grown in 1998 and 1999. Tubers from plants of Dali and Oleva were harvested three and four times during the final growth period, respectively. The tubers were analysed for blackspot susceptibility and colour of blackspots four times during storage and the tuber cortices were analysed for discolouration potential, activity of polyphenol oxidase (PPO) and concentration of PPO substrates (free tyrosine, chlorogenic acid, caffeic acid). During tuber growth, an increasing abundance of red- and yellow-coloured intermediates and final black-coloured compounds in the assay of potential discolouration were observed. The overall correlation coefficients (r) between red intermediates and free tyrosine were 0.74 and 0.64 for Dali and Oleva, respectively, whereas they were 0.49 and 0.68, respectively, between yellow intermediates and free tyrosine. The overall correlation coefficients between black end-products and free tyrosine were 0.69 and 0.67 for Dali and Oleva, respectively. No overall correlation was found between the discolouration potential and other phenols. Changes in blackspot susceptibility and in colour of blackspots during growth and storage of the tubers could not be ascribed to changes in the discolouration potential (colour development of blended tuber tissue), PPO activity or concentration of phenols in the tuber cortices. Contrary to the discolouration potential, the blackspots were significantly darker at the beginning of the storage period compared to later in storage. Furthermore, during tuber growth the stable colour of bruises did not reflect the concomitant increase in discolouration potential. The results provide supporting evidence that intracellular compartmentation is the determining factor for blackspot bruise susceptibility in potato tubers.

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