Ultrastructural Localization of Ions

Abstract

The cytoplasmic distribution of Cl− in Nitella translucens was assessed by means of two contrasting approaches. The first involved the histochemical precipitation of Cl− with Ag+ followed by X-ray analytical verification of the silver precipitation products, and the second, quench-freezing of whole Nitella cells followed by freeze-substitution in the presence of silver under anhydrous conditions. Both methods produced identical evidence for Cl− distribution, showing that a large proportion of the Cl− is present in the stationary cortical gel layer which includes the chloroplasts. However, the chloroplasts appeared to be low in Cl− content while the bulk of the Cl− appeared to be situated between the chloroplasts and plasmalemma. Experiments were carried out in other to detect the pathway of the ‘fast component’ of halide ion transfer to the vacuole. Br− was supplied for various time intervals to low Cl−Nitella cells, followed by attempts to differentiate between AgCl and AgBr deposits. Solutions of various strength of NH4OH or ammonium carbonate were used to remove AgCl (but not AgBr) deposits by formation of a Ag(NH3)2+ complex. Although X-ray analytical verification showed that the method had some potential usefulness it could not be carried out successfully because of loss of structural detail caused by NH+4. The distribution and density of deposits near the plasmalemma suggested the occurrence of a process in which cytoplasmic loading is achieved by a sequential rupture and repair of the plasmalemma membrane. Vesicles and reticulate structures in the streaming cytoplasmic phase generally showed very little deposit, but these structures, together with the tonoplast, became greatly enriched with deposits when cells had been given a brief exposers (3 min) to a Cl− or Br− solution. These rapid changes may possibly be related to the ‘fast component’ of halide ion transfer to the vacuole.