[Active and inactive phosphate uptake in leaf cells of Elodea densa at high external phosphate concentrations].

The phosphate uptake in the leaf cells of Elodea densa shows multiple isotherms in the range [S]>1 mmole P/l to 100 mmoles P/l. In the dark the uptake isotherms contain three distinct parts (II/1, II/2 and II/3); the first two obey Michaelis-Menten kinetics, whereas the third is exponential. In the light the phosphate uptake curve consists only of two parts (II/1 and II/2) agreeing with Michaelis-Menten kinetics, the exponential part being absent.Cellular phosphate content was found to be 45 mmoles/l. Data concerning the membrane potential E for Elodea densa were obtained from Jeschke (1970). In accordance with the Nernst equation a change from the hyperbolic curve to an exponential one was expected at a concentration of about 60 mmoles P/l in the dark and at above 100 mmoles P/l in the light. The results obtained agree with these theoretical calculations: in the dark, the change from the hyperbolic to the exponential curve was observed at [S]=50 mmoles P/l, which is in electro-chemical equilibrium with the cellular orthophosphate content of about 35 mmoles/l (inorganic P content amounting to 80 per cent of total phosphate). In the light no change towards an exponential curve was noticed.The effect of the uncoupler CCCP in the light and in the dark was examined in order to elucidate its influenc on (32)P incorporation into the fractions of inorganic, organic and acid-insoluble phosphates, the inorganic fraction representing phosphate uptake. The inhibition of the uptake into the inorganic part decreases with an increasing inactive component of total uptake, while the fixation in the organic fraction is severely curtailed at all concentrations tested. The acid-insoluble fraction remains unaffected.