SUMMARY: Mycelium of potassium-limited had a lowered potassium content and an increased sodium content compared with mycelium grown in presence of excess potassium and no sodium. Potassium uptake involves the exchange for Na and probably also for H.

Total potassium uptake was decreased to half by a fivefold equivalent excess of rubidium, but was essentially unaffected by a 100-fold equivalent excess of sodium. Calcium and magnesium had intermediate effects. Potassium uptake was inhibited by sodium azide (m) and by dinitrophenol (0.3 m). These depressed the equilibrium potassium level but had a much smaller effect on the time required to reach equilibrium. Loss of potassium by K-loaded mycelium to sodium azide or dinitrophenol solutions or to water was small. Much greater losses occurred to solutions of rubidium but not of sodium.

It is suggested that the equilibrium potassium level is determined only in part by exchange-diffusion at the mycelium surface, and that net potassium uptake is a metabolic process.


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