Localization of condensed tannins in apple fruit peel, pulp, and seeds

Abstract

Seven varieties of post-harvest apple (Malus domestica Borkh.) fruit, i.e., Spartan, McIntosh, Newtown, Royal Gala, Jonagold, Red Delicious, and Golden Delicious, were examined for the presence and location of condensed tannins in the peel, pulp, and seeds. Statistically significant differences in the condensed tannin content were found between varieties and sources, and in different tissues within a variety. Red Delicious had the greatest amount of condensed tannin in the peel and pulp, whereas Golden Delicious had the least. The amount in the seed was low and variable. Light and electron microscopy revealed tannins in the hypodermal layers of the peel, near the floral bundles in the cortex of the pulp, and in the thick-walled sclerenchyma and crushed parenchyma of the testa. The condensed tannin deposits in the peel appeared most concentrated in the hypodermal cell layer adjacent to the epidermis, with lesser amounts in cell layers closer to the cortex. Electron micrographs revealed that condensed tannin deposits were always found in the vacuole, taking different forms, and mainly associated with the tonoplast. Key words: condensed tannins, apple, peel, pulp, seed, vanillin–HCl, microscopy.