Critical evaluation of methods for estimating the vacuolar pH of plant cells

The vacuoles of higher plant cells have been shown to be the site for accumulation of ions and metabolites (Matile 1978; Leigh et al. 1979, Leigh 1983; Marty et al. 1980; Boudet et al. 1984). The mechanisms involved in the storage functions of vacuoles are not well known. However, the presence of ATPases on the tonoplast of a number of plant species has been demonstrated (d’Auzac 1977; Lin et al. 1977 ; Leigh and Walker 1980; Cretin 1982; Aoki and Nishida 1984). These ATPases are considered to be responsible for creating a proton electrochemical gradient across the tonoplast. A classical idea is that this gradient could be used for the transport of ions and metabolites and their accumulation in the vacuole. The proton gradient across the tonoplast can also be directly used for the accumulation of secondary plant products diffusing through membranes as neutral molecules and accumulated as cations. This is the case for some indole alkaloids (Renaudin and Guern 1982) and nicotine (Kurkdjian 1982).