The ultrastructure of germinating barley seeds. II. Breakdown of starch granules and cell walls of the endosperm in three barley varieties

Abstract

The breakdown during malting of starch granules and cell walls of the endosperm, as well as the morphological changes occurring in the scutellum and the aleurone layer, have been examined by thin sectioning and freeze-fracturing in the barley varieties Nordal, Minerva and Klages. Intact starchy endosperm cells contain large and small starch granules embedded in a matrix. In most cases the fracture plane exposes the smooth surface of the starch granules and occasionally adhering material, which probably consists of remnants of the amyloplast envelope. Degradation of the endosperm tissue during malting starts at the embryo end, and the transitional zone between intact and degraded endosperm is approximately one cell layer wide. The degradation of the large starch granules affects at the beginning only their outer edge. Subsequently, corroding channels form towards the center of the granules and elicit a rapid breakdown of the central part. Further degradation proceeds from the center giving rise to hollow shells, which finally break up into pieces. It is concluded that the susceptibility to enzyme attack increases towards the center of the large starch granules of the barley endosperm. The saw-toothed pattern observed in the corroding channels indicates, in agreement with earlier studies, that the enzyme susceptibility varies periodically in a radial direction. The three barley varieties examined differed in malting quality, but the differences were not reflected qualitatively in the ultrastructure of the scutellum, aleurone layer or the endosperm during malting.