HISTOLOGICAL AND HISTOCHEMICAL CHANGES IN DEVELOPING AND RIPENING PEACHES. II. THE CELL WALLS AND PECTINS

REEVE, R. M. (U.S.D.A., Albany, California.) Histological and histochemical changes in developing and ripening peaches. II. The cell walls and pectins. Amer. Jour. Bot. 46(4): 241-247. Illus. 1959.-Histological and histochemical observations on developing and ripening clingstone and freestone peaches have revealed that, after cell divisions have ceased in the mesocarp, cell wall thickening and cell enlargement in the mesocarp parenchyma increase until the fruit is nearly full cell size. The cell walls then decrease in thickness as the fruit ripens and softens. Degree of methyl esterification of the pectic substances, as estimated histochemically, remains at about 75-80%o in immature fruits during their cell-enlargement phase of growth. Percent of methyl esterification apparently is much lower, or amounts of esterified pectates are very low during the meristematic phases of fruit growth. Just prior to ripening, degree of esterification increases and approaches 100%o in hard,. ripe fruit at about the same time that the parenchyma cell walls exhibit their greatest thickness or degree of hydration. The degree of esterification of the pectic substances then rapidly decreases and the cell walls become appreciably thinner as the ripening fruit softens. Further relation of these changes in wall thickness, in degree of esterification of the pectins, and in other cell wall carbohydrates to the textural qualities of ripening fruits is discussed. Interpretations concerning cell wall plasticity, cell growth and relation between auxin and changes in pectins also are presented. APPARENT RELATION between changes in catechol tannins and the differentiation and growth of the mesocarp parenchyma and maturation of endocarp sclereids in peaches has been described previously (Reeve, 1959). The present communication describes further observations on the mesocarp of growing peaches with particular reference to changes in the parenchyma cell walls and their pectic substances. As reviewed by Heyn (1940), cell growth requires a plastic condition in the cell wall. Changes in the pectins and other cell wall materials influence the physical properties of the wall and are thus significant to the physiology of cell growth (Bonner, 1950). The role of pectic substances in changes of texture upon the ripening of fruit also has been accorded much practical significance since the work of Branfoot (1929). Conversion from insoluble to soluble forms of pectin during the ripening process has become generally accepted as one of the important changes of these substances. Addoms et al. (1930) employed Branfoot's techniques in a study of developing and ripening free. stone ('Elberta') and clingstone ('Shipper Cling') peaches. Only slight differences between these 2 kinds of peaches were reported to occur in the physical state of the pectins in situ during fruit ripening. They were noted principally in the skin area where, in the ripe 'Elberta,' the middle lamella pectins of the hypodermal area became comparatively more soluble so that the skins were more readily detached than in the ripe 'Shipper Cling.' These workers further considered that the development of 1 Received for publication November 20, 1958. thinner cell walls and actual physical breakdown of some of the mesocarp parenchyma cell walls in the ripe 'Elberta' were the principal cause of the soft, "melting flesh" texture of the freestone in contrast to the thicker and completely intact cell walls of the ripe, firm-textured clingstone peach. The present investigations do not substantiate this physical cause of texture distinction between several varieties of California-grown clingstone and freestone peaches. Likewise, no marked differences between clingstone and freestone varieties were found in histochemically detected changes of the pectin in situ during growth and ripening of the fruit. The relation of these results to other recent findings on cell wall substances and growth physiology is discussed in this paper. MATERIALS AND HISTOLOGICAL METHODS.-The peaches periodically collected for studies on tannins were used also for the present investigations and all fresh sections were cut on a sliding microtome as previously described (Reeve, 1958). Cell diameter and cell wall thickness were measured for the developing mesocarp parenchyma using a calibrated screw-micrometer eyepiece. An 8-mm. dry objective was used for cell diameter measurements and only the spherically shaped parenchyma cells of the outer mesocarp regions were measured. Each measured sample consisted of 500 or more cells. A 3-mm. (57X) dry objective was used for measuring the cell walls in the outer mesocarp region. The top cut surfaces of fresh sections were searched for suitable intercellular spaces where 2 or more single walls could be measured. The "V" in the center of the eyepiece scale was centered at