The Effect of Nitrogen on the Growth and Sugar Content of Sugar-beet

Abstract

Nitrogen fertilizer applied to sugar-beet increased plant and root dry weight and leaf area, and decreased the sugar content of the roots per cent of both fresh and dry weight. Change in leaf area accounted wholly for the increase in plant dry weight produced by nitrogen, because net assimilation rate was unaffected. Nitrogen did not alter the partition of the total assimilate between roots and shoots, but increased the fraction of total assimilate entering the roots that was used in growth, at the expense of that stored as sugar. Thus, plants with more nitrogen had a smaller proportion of their root dry weight as sugar because more was metabolized in growth of the roots, and not because less entered the roots. The heavier roots of plants given more nitrogen were larger in cross-sectional area because the areas of both parenchyma and vascular zones of each peripheral ring within the root were larger; the number of rings was not increased. Nitrogen increased the areas of the tissues in these zones by enlarging cell volumes, not by increasing the number of cells within the tissues. Increase in cell volume was accompanied by proportional increases in the weights of non-sugar dry matter per cell and water per cell, but the amount of sugar per cell was proportional to cell volume only during the initial stage of cell expansion up to cell volumes of about 15 X io~8 cm3; thereafter it was less than proportional, so that sugar per cent of both fresh and dry weight decreased as cell size increased beyond 15 X io~8 cm3. The relation of sugar per cell to cell volume was the same with both amounts of nitrogen given. This implies that increase in nitrogen supply made the sugar concentration of the root less by increasing the size of the root cells and not by a specific effect on sugar storage.