Translocation of Nitrogenous Substances in the Cuthbert Raspberry

Abstract

1. Data are presented concerning the distribution and translocation of total organic, total soluble organic, alpha amino, diamino, amide, ammonium, and nitrate nitrogen in the vegetative canes of the Cuthbert raspberry. Double rings were used to check the movement of nitrogenous substances, one ring 15 cm. and the other 30 cm. above the ground. The plants were cut for analysis into 15 cm. lengths. Leaves were analyzed separately from stem segments. 2. There was a steady increase in the concentration of total organic nitrogen from the basal segment to the top. Total soluble organic nitrogen and alpha amino acids attained a maximum concentration in the subterminal segment. Total organic nitrogen was higher in the leaves than in the stem, but the total soluble nitrogen and amino acids were about 50 per cent lower in the leaves than in the stems. The concentration of these substances was about 50 per cent lower in the terminal than in the subterminal stem segment. Protein was therefore most abundant in the stem top and leaves. 3. Nitrogenous compounds did not accumulate in the upper regions of the ringed plants. There was not a significant accumulation below the rings. Increase in the concentration of soluble nitrogen in the necrotic segment between the rings was owing to production of amino acids by proteolysis. 4. All nitrogen gradients were positive, and are thought to be little more than manifestations of the ratio of the amount of living, protoplasm-containing cells to non-living, protoplasm-lacking cells in the various parts of the plant. Thus the concentration of total organic nitrogen is greatest in the top segment and leaves, where there is least differentiation and thickening of cells arid much meristematic tissue. The gradient of soluble nitrogen from subterminal to top segment (positive upward), and from stem to leaves (positive laterally), seems to be a manifestation of the differential utilization of the soluble forms, with the greater amount of protein synthesis taking place in the top and leaves, rather than an indication of movement of nitrogen. The gradients in the stem (positive downward) are the result of synthesis or accumulation of nitrogenous substances in the living cells. 5. There appears to be considerable loss of nitrogen from the above-ground portion of the plant from May 15 to June 23. Since there is no accumulation above the rings, the 16-22 per cent loss cannot be explained by movement to the roots in the bark. It is hardly conceivable that it was translocated to the roots in the xylem, in which the transpiration stream is under a great negative tension owing to a high rate of transpiration. It is suggested that the loss is in the form of a gas, or occurs through leaching by rain. 6. It is concluded that there is no definite longitudinal translocation of organic nitrogen either upward or downward in the bark. Nitrate is the important translocational form of nitrogen in the Cuthbert raspberry. It rises freely in the xylem, and is available for reduction and elaboration to amino acids and protein not only in the leaves but in all living cells of the plant where conditions favorable to reduction exist. The amount of nitrate reduction and elaboration is proportional to the amount of protoplasm in the various parts of the plant; thus the greatest amount of protein is found in the stem top and leaves. Reduction and synthesis take place also in the stem and in proportion to the concentration of protoplasm per unit of weight; hence the existence of a positive concentration gradient. 7. In the stem the phloem is a highly metabolic tissue, and absorbs nitrates and other inorganic salts from the relatively passive xylem. The presence of inorganic solutes in the phloem is the result of an absorption process homologous with the absorption of salts by roots, and is not a manifestation of primary solute translocation.