164 MINERAL SALTS ABSORPTION IN PLANTS 



that the constituent proteins are mobile, and probably move 

 periodically from the surface into the bulk of the cytoplasm carrying 

 bound ions with them. Pinocytosis may play an important part 

 in this process. Within the cytoplasm, proteins are continually 

 being broken down and reformed, a process — referred to as "turn- 

 over" — which involves attachment of the protein to a template or 

 former which is believed to be a molecule of ribonucleic acid (RNA). 

 When a protein molecule complexes with RNA it is probable that 

 some or all of the bound ions are released. Release of ions may occur 

 predominantly in localized regions of the protoplasm which are rich 

 in RNA. Such a group of free ions may then attract water from their 

 surroundings to form a small aqueous vesicle containing a con- 

 centrated solution of salt. Such vesicles are a prominent feature of 

 cytoplasm when viewed with the electron microscope. 



Transfer of salts from these small vesicles into the central 

 vacuole may occur by periodic fusion of the vesicles with the 

 vacuole. According to this view, a carrier mechanism does not 

 operate across the tonoplast, although the vacuolar membrane 

 contributes to the overall resistance of the cytoplasm to movement 

 of ions across it. 



The suggested hypothesis has some features in common with 

 other proposed mechanisms of salt absorption. The mechanism 

 suggested by Goldacre and Lorch (Figure 12d, p. 43) for example, 

 also depends on the ion-binding capacity of proteins, and Bennett 

 (Fig. 12e) proposed that membrane flow and vesiculation may be 

 involved in transferring bound salts into and through cytoplasm. 

 Steward (Fig. 31a, p. 87) has pointed out that nucleic acids may be 

 linked to salt absorption through their function in protein synthesis. 

 Robertson (1951) suggested that mitochondria might transfer ions 

 into the central vacuole of a parenchyma cell, by bursting at the 

 vacuolar boundary in just the same way that vesicles are thought to 

 do in the hypothesis outlined above. 



The best evidence supporting the view that salt absorption is 

 linked to protein turnover, other than by a common dependence on 

 respiratory energy, comes from the observation that when protein 

 synthesis and turnover are inhibited with chloramphenicol, accumu- 

 lation also stops, without respiration necessarily being affected. 

 This substance may exert its effects at the stage when the protein- 

 nucleic acid complex is formed and ions are released into the 



