METHODS OF DETERMINING GLUCOSE AND FRUCTOSE IN CORN TISSUES

Abstract

Introduction In a series of studies on the physiology of corn, considerable attention has been given to the concentration of sugars in the different tissues of the plant, especially in relation to seasonal development. In these studies the total sugar content was differentiated into free reducing sugars and sucrose. It seemed desirable to carry the investigations a step further and determine the proportions of glucose and fructose which make up the free reducing sugars. Due to the similarity in chemical properties of these tw-o sugars, however, procedures have not been standardized for the determination of either one of them in the presence of the other and sucrose in plant tissues. It consequently was necessary to make a preliminary survey of the avail-able methods and to work out procedures which might be satisfactory for use with samples of corn tissues. Three of the general methods which have been used for determining glucose and fructose in mixtures of glucose, fructose, and sucrose are: (1) the oxidation of glucose in alkaline solution with iodine, (2) the oxida-tion of fructose with Nyns' solution, a cupro-potassium carbonate solution, and (3) the method of analyzing a mixture of three sugars by determining the combined reducing power, polarization, and the direct determination of sucrose. Materials and methods In our studies of corn the tissue to be sampled was prepared by passing it through a food grinder, and it was desired to develop methods which might be applicable to two kinds of sampling. In one, the "Expressed sap method," the sap was expressed from 100 gm. of the ground tissue in a laboratory hydraulic press at a pressure of 5000 lb. per sq. in. Fifty-ml. aliquots of the sap were preserved by boiling with about 150 ml. of 95 per cent. alcohol and then making up to 200 ml. when cooled to room tempera-ture. From determinations of the weight, total solids content, and specific gravity of the sap and the total moisture content of the tissue, the quanti-ties of sugars found in the sap were calculated to percentages of fresh tissue. 1 Based on investigations co-operative between the Office of Cereal Crops and Dis-eases, Bureau of Plant Industry, U. S. Department of Agriculture, and the Department of Agronomiy, Ohio Agricultural Experiment Station. 47 PLANT PHYSIOLOGY The second kind of sample was obtained by the standard method of extracting 50 gin. of the ground tissue with 80 per cent. alcohol in a Soxhlet extractor. As the first method is quicker and simpler, the primary purpose of the extraction method was to check the accuracy of the expressed sap method. IODOMETRIC DETERMINATION OF GLUCOSE An excellent review of the literature and a critical study of the factors involved in the oxidation of aldose sugars, such as glucose, by iodine in alkaline solution has been reported recently by ILINE and ACREE (4). The reaction is usually conceived of as occurring as follows: RCHO + 3NaOH + I2 -> RCOONa + 2NaI + 2H20