Ammonium Triggers Uptake of NO−3 by Chenopodium rubrum Suspension Culture Cells and Remobilization of their Vacuolar Nitrate Pool

Abstract

Photoautotrophic cell suspension cultures of Chenopodium rubrum require high concentrations of nitrate and ammonium. During the growth phase total NH₄⁺ and the greater portion of NH₃⁻ were consumed. During the stationary phase nitrate uptake continued but at a substantially smaller rate than during the growth phase. During growth the bulk of the absorbed N was incorporated into protein, the amount of which was then maintained constant until senescence. NH₃⁻ was accumulated upon transition between the growth and the stationary phase. NH₃⁻, like the free amino acids, was deposited in the vacuole but, unlike these compounds, could not be remobilized upon transfer of the cells into N-free medium. Readdition of NH₄⁺ to the medium, however, resulted in a mobilization of the vacuolar NH₃⁻-pool. Reutilization of both vacuolar N-storage pools must have been accomplished by recycling from the vacuole to the cytoplasm because N-metabolizing enzymes could not be detected in isolated vacuoles. Transfer of the cells of the stationary phase into medium containing NH₃⁻ and NH₄⁺ resulted in an induction of nitrate uptake by the cells, but only after a lag phase of 4–5 days. It is conceivable that NH₄⁺ induces NH₃⁻-translocating systems in the plasmalemma and in the tonoplast.