RAFFINOSE METABOLISM IN GERMINATING BARLEY

Abstract

The trisaccharide raffinose is widely distributed throughout the Spermaphyta (French, 1954), not infrequently accounting for a substantial proportion of the free sugars of seeds, rhizomes or roots. Structurally, raffinose may be represented as a-D-galactopyranose (I -6) a-D-glucopyranose (I--2) ,-D-fructofuranose; oligosaccharides related to raffinose include the tetrasaccharide stachyose which contains a second galactose unit joined by an a (I-%6) linkage to the terminal galactose residue of raffinose, and the pentasaccharide verbascose. These oligosaccharides resemble one another in being terminated by sucrose and in containing a number of a (i --> 6)-linked galactose units. In barley the only one of this group of oligosaccharides which has been identified with certainty is raffinose itself. First recorded from ungerminated barley by O'Sullivan (i886), raffinose in cereals has attracted considerable interest and numerous analyses are now available for the total quantities of the sugar present in the entire grain (see, e.g. MacLeod, I952) and in the isolated embryos (James, 1940). The average amount of raffinose in intact barley corns is of the order of 0.5 per cent of the dry weight, though figures ranging from 0. 14-0.83 per cent have been recorded. Raffinose normally accounts for about one-quarter of the total free sugars and low-molecular fructosans of the ungerminated grain. The distribution of the sugar within the corn is not, however, uniform. Thus, Colin and Belval (1933) could not detect raffinose from barley flour, though the embryo contained 4 per cent of the sugar, and MacLeod (1952), in an examination of differentially-milled fractions of barley, reported 3.6 per cent of raffinose from the fraction containing the germ, compared with only o.oo8 per cent from the pearled grain, which is almost exclusively endosperm. In barley, therefore, raffinose appears to be associated predominantly with the living meristematic tissues of the embryo. When barley germinates, raffinose disappears from the developing seedling. Thus Kluyver (1914) failed to detect raffinose from partly grown barley malt, though this sugar accounted for 0.45 per cent of the unmalted grain, and James (1940), in a detailed analysis of sugars from excised embryos and seedlings of Plumage-Archer barley, found that raffinose had completely disappeared from the young plant after 24-hour growth at 2I C. On the other hand, MacLeod, Travis and Wreay (I953), using chromatographic methods of separation to follow changes in the sugar content of Ymer barley during commercial malting, were able to demonstrate the continued presence of raffinose for approximately 4 days after the wetting of the grain. A similar pattern of slow utilization of raffinose is also apparent in results quoted by Harris and MacWilliam (I954) for a commercially-malted Spratt-Archer barley. The delayed mobilization of raffinose during malting compared with the very rapid disappearance of the sugar from laboratorygrown material was initially attributed to the lower temperatures prevalent during the