Cotyledon, galactomannans

Fig. 1.—A Cross-section of the Outer Part of a Seed of Trigonella foenum-graecum Before Mobilization of the Galactomannan, Showing the Three-layered Seed-coat (S) and a Small Part of the Cotyledon (C), with the Endosperm in Between. [The aleurone layer (A) is the outer cell-layer of the endosperm, and the rest of the endosperm is composed of large cells that have thin, primary walls and are completely filled with the dark-stained galactomannan (G). Stained with the periodic acid-Schiff reagent x300 (reproduced, by permission, from Ref. 199).]...

It has been known for some time that the albuminous seeds of the family Leguminosae contain, in the endosperm, galactomannan as the reserve polysaccharide. The exalbuminous seeds, on the other hand, have the main reserve in the cotyledon in the form of starch, oil, and proteins.198,199 The seed galactomannan-reserves are generally mobilized during the process of germination.43,57,81,82,199-205 Detailed study... 

Fig. 4.—Disappearance of Galactomannan from the Endosperm, and Appearance of Starch in the Cotyledon, During Germination of Seeds olTriganella foenum-graecum that had Already Been Soaked for 24 Hours at 24-26°. (Adapted from Figs. 5 and 6 of Ref. 199.)...

During the third, and final, phase (24 h following the second phase), galactomannan is absent (see Fig. 5), and the endospermic carbohydrates of low molecular weight also disappear completely and rapidly. Cotyledons, however, have a high level of starch (see Fig. 4). 

Seeds other than cereals Parenchymatous (e.g., cotyledons of peas) and some cells with thickened endosperm walls (e.g., guar) Same as parenchymatous tissues of dicotyledons Galactomannans and small amounts of cellulose, pectic substances, and glycoproteins... 

Finally a word about the endospermic legumes. As we have mentioned previously, the galactomannan reserves of the endosperm of these seeds might be more important for retention of water within the seed to allow early embryo growth than for the provision of nutrients. The cotyledons of these legumes are usually rich in fats and protein but their mobilization does not appear to have received any attention. 

The jS-D-mannanase activities in germinating seeds of lucerne, guar, carob, and soybean could be separated from the accompanying a-D-galactosidase activities only with difficulty. The role of the jS-D-mannanases in degrading the galactomannan in cotyledon embryos was discussed. 

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