Devlopmental Physiology of Sugar-Beet: II. EFFECTS OF TEMPERATURE AND NITROGEN SUPPLY ON THE GROWTH, SOLUBLE CARBOHYDRATE CONTENT AND NITROGEN CONTENT OF LEAVES AND ROOTS
Plants were grown at temperatures of 15 and 25 °C with two rates of nitrogen supply. The changes in dry weight, leaf area, cell number, mean cell volume, soluble carbohydrate, and total nitrogen concentration of the cotyledons, the first and second pair of true leaves, and the storage root were measured. Changes in cell number and cell volume of the first pair of true leaves and storage root of plants were also measured at 11, 18, 25, and 32 °C. Leaf growth before unfolding was chiefly by increase in cell number and after unfolding by increase in mean cell volume, while the growth of the storage root was almost entirely by increase in cell number. The rates of cell division and cell expansion were fastest at 25 °C, but the initially high rates of cell division in the terminal bud and in individual leaves decreased rapidly and greater rates were maintained at the sub-optimal temperatures, i.e. 15 and 18 °C. After an initial period of slow growth, the first-formed leaves grew faster and became larger at 15 than at 25 °C. Leaves were produced, unfolded, grew faster, and became larger with increase in the external concentration of nitrogen, because cells divided and expanded faster, so that nitrogen increased the number and size of cells. Sugar concentration was greater at 15 than at 25 °C in leaves but not in the storage root. Sugar concentration in the petioles of the first and second pair of true leaves increased to 1.2 and 2.0 per cent fresh weight respectively. Decreased nitrogen supply temporarily increased the sugar concentration of cotyledon petioles and the seedling hypocotyl, but later decreased it in the leaves and storage root. Nitrogen concentration was greater in the leaves and storage root at 15 than at 25 °C with the larger nitrogen supply. Nitrogen concentrations were similar in young leaves of all treatments but as the size of leaves increased nitrogen concentrations decreased most rapidly at 25 °C with the smaller nitrogen supply. It is suggested that when increased leaf production and storage-root growth occurs at temperatures below the growth optimum (25 °C), they may be due to an effect of increased carbohydrate supply on cell division and sugar storage.