Determination of Total Available Carbohydrates in Plants.

The estimation of carbohydrates plays an important part in both pure and applied plant physiology. While satisfactory methods of extracting and determining the individual types of carbohydrates are available, the complete fractionation of these is a tedious process. Furthermore, for many purposes of a more applied nature the determination of the total available carbohydrate content is of greater significance than that of individual carbohydrates or groups of carbohydrates. The term "total available carbohydrate" may be defined as including all those carbohydrates which can be used in the plant body as a source of energy or as building material, either directly or indirectly after having been broken down by enzymes. In most ordinary, higher green plants the bulk of available carbohydrate is composed of sugars, fructosans, dextrin and starch, whereas hemicelluloses and true cellulose act merely as structural materials and as such cannot further be utilized in the same way as the former (1, 6, 7, 8, 9, 11). Apparently there is no satisfactory method of assessing the total available carbohydrate content by means of a single determination. Acid hydrolysis of entire samples has occasionally been used as a means of determining total "hydrolyzable" carbohydrate. The results of such determinations, however, are bound to be inaccurate and even erroneous, because it is impossible to separate the starch fraction from the structural carbohydrates in this way, and also because the relatively severe conditions of hydrolysis required for the complete breakdown of starch and dextrin result in the destruction of most of the free and combined fructose (5).