EFFECTS OF CA2+ AND CD2+ ON THE CARBOHYDRATE-METABOLISM IN SUGAR-BEET (BETA-VULGARIS)

Sugar beet (Beta vulgaris L. cv. Monohill) were cultivated in a nutrient solution with different combinations of Ca2+ (36, 180, 720 or 3560-mu-M) and Cd2+ (0, 1, 5 or 20-mu-M). The dry and fresh weights, the content of Ca2+ and Cd2+, sucrose, fructose, glucose and starch in 5-week-old plants was analysed as well as the rate of [C-14]-sucrose uptake in discs from 3-month-old storage roots. The carbohydrate metabolism was indirectly affected by the presence of calcium or cadmium. Cadmium caused a diminished dry weight and carbohydrate concentration. The dry weight was unaffected by the Ca2+ level but the carbohydrate distribution between storage and growth processes was affected; at low Ca2+ in the tissue, the growth was retarded and the level of storage carbohydrate increased, while at high Ca2+ the opposite was found. The [C-14]-sucrose-uptake decreased in tap roots cultivated at low Ca2+. Long term exposure to Cd2+ also decreased the sucrose uptake in tap roots. Direct Cd2+ addition to the assay medium, however, increased the sucrose uptake, probably at the tonoplast, while Ca2+ had no transient effect on the uptake. Cadmium increased the Ca2+ concentration in the plant, but Ca2+ did not affect the net-uptake of Cd2+.