Biochemical Potential of Potato Tubers To Synthesize Blackspot Pigments in Relation to Their Actual Blackspot Susceptibility

In a study of six potato varieties (Solanum tuberosum L.), which were grown under two different fertilization regimes, the biochemical potential of tuber tissue to synthesize blackspot pigments was distinguished from the actual blackspot susceptibility exhibited after impact application. Blackspot susceptibility and biochemical potential for pigment synthesis were not correlated, which supports the hypothesis that the extent to which the blackspot potential is actually being used is mediated by the sensitivity to cell decompartmentation. Quantification of polyphenol oxidase (PPO), soluble protein, and endogenous PPO substrates demonstrated that the content of free tyrosine is the predominant determinant for the biochemical potential for blackspot synthesis. PPO was not a limiting factor for pigment synthesis, even if PPO content was relatively low. It was therefore concluded that manipulation of PPO activity may offer a solution to the problem of blackspot formation only if it leads to a virtually complete elimination of PPO activity. Keywords: Solanum tuberosum L.; Solanaceae; potato; blackspot; bruise; melanin; tyrosine; chlorogenic acid; caffeic acid; polyphenol oxidase; enzymic browning