HISTOLOGICAL AND HISTOCHEMICAL CHANGES IN DEVELOPING AND RIPENING PEACHES. I. THE CATECHOL TANNINS

REEVE, R. M. (U.S.D.A., Albany, California.) Histological and histochemical changes in developing and ripening peaches. I. The catechol tannins. Amer. Jour. Bot. 46(3): 210-217. Illus. 1959.-Selective histochemical tests for catechol derivatives revealed marked changes in tannin contents during the development and ripening of peaches. The principal test used was a nitroso reaction found to be colorimetrically selective for catechol and derivatives such as chlorogenic acid. Intensities of color produced by this test were photometrically measured. Increases in catechol derivatives, so indicated in situ, were associated with early maturation and lignification of the endocarp sclereids and particularly with cessation of cell divisions and early cell enlargement of the mesocarp parenchyma. Characteristic progressive localizations of tannins in patches of enlarging mesocarp parenchyma cells, as revealed by staining with iron salts as well as by the nitroso reaction, were observed in green fruits approaching maturity. The insoluble tannin color complexes produced by these tests remained coarsely granular and intense in green fruit. Upon ripening of the fruit color intensity produced by the nitroso reaction decreased and the insoluble tannin complexes were more finely divided. The association of phenols with cellular differentiation, lignification and suberization are briefly discussed. INTEREST IN TANNINS and astringency in fruits and vegetables dates from the beginning of botanical inquiry. Astringent properties of substances in oak galls, chestnuts, species of Acacia, sumac and other plants were recognized and described by Dioscorides (ca. 100 A.D.) who also described one of the earliest chemical tests-the detection of iron vitriol with the juice from gall-nuts. Albertus Magnus (1193-1280 A.D.) was one of the first to associate astringency with tannins by his comparison of the "styptic" flavors of some plants with that of the gall (Nierenstein, 1935; Reed, 1942). Distribution of tannins and the relation of changes in their physical state to loss of astringency upon fruit ripening have been described in modern times for bananas and persimmons (Bigelow et al., 1906; Howard, 1906; Lloyd, 1911a and b; Barnell and Barnell, 1945). Tannin content and astringency also are known to be influenced by growing area, as exemplified by peaches (Guadagni and Nimmo, 1953). The tannins, however, are a complex assemblage of polyhydroxylic phenols which often have no close structural relationship. They usually are distinguished from other phenols by their precipitating action on proteins, but this characteristic does not apply to the caffetannins such as caffeic acid and chlorogenic acid (Russell, 1935). Phenols and their derivatives appear to be universal in plants, but their exact functions in various processes are often obscure. They apparently originate from sugars, amino acids and other simple compounds and many become elaborated into complex flavonoids and glycosides (Geissman and Hinreiner, 1952). Relation between tannis and enzymatic browning in fruits was first demonstrated by Onslow (1920) 1 Received for publication September 16, 1958. who identified catechol tannins as substrates for the oxidizing enzymes in several fruits. Recent studies have indicated that chlorogenic acid may be one of the principal catechols involved in fruit browning (Ingraham and Corse, 1951; Weurman and Swain, 1953). As discussed by Van Fleet (1952, 1954), the actions of phenolases and other oxidizing enzymes with phenolic and lipoidal substrates are associated with the development of specialized tissues, such as the endodermis, and phenol-quinone systems appear to be intimately related with early differentiation in root and stem tissues. Relatively little has been known about comparable associations of phenolics with cellular growth and differentiation in fruit tissues which, because of their temporary status in the life cycle of a plant and their senescence upon ripening, present a physiological situation different from that of the vegetative tissues. This communication describes the results of histochemical tests made on the catechol tannins of several varieties of peaches during their growth from the soft pit stage to fully ripe, soft fruit. A semi-quantitative application of the histochemical test for catechol tannins, other histological features of growth, ripening and associated changes in the pectice substances of the cell walls will be discussed in subsequent papers. MATERIALS AND HISTOLOGICAL METHODS.Peaches were collected at intervals of 1-2 wk. during the 1954, 1955 and 1957 seasons from the University of California orchard near Winters, California, beginning about 1 month after bloom.2 Varieties collected included the freestone peaches 2 The cooperation of Dr. C. 0. Hesse, Chairman of the Department of Pomology, University of California at Davis, California, in granting permission to make these collections is greatly appreciated.