Light-induced changes of cytosolic pH in Eremosphaera viridis: recordings and kinetic analysis

ATPase in the plasmalemma.Thus recovery from acid load in the cytosol can be performed by proton transloca-Short-term light-induced pH changes were measured tion across one of the adjacent membranes, tonoplast or by means of pH-selective microelectrodes in the uniplasmalemma, or, alternatively, by a biochemical pH-stat cellular green alga Eremosphaera viridis.Cytosolic pH mechanism.It is still unknown which mechanisms contribchanges were always transient and reversible within ute to the pH-stat mechanism under varying conditions.1-3 min.Taking into account the low-pass filtering byOne way to approach this question is to investigate the the experimental set-up, the light-dependent cytosolic light-induced changes of cytosolic pH.They provide a pH changes could be described by a sum of two expophysiological approach to disturbing cytosolic pH and to nential functions.By mathematical analysis it is deminvestigate short-term pH regulation.These changes have onstrated that the transient nature of light-dependent been observed in a number of plant species and share pH changes was due to a consecutive pH-stat common characteristics.Darkening of green plant cells regulation and not to a parallel light-triggered reaction.induces a transient acidification and illumination causes The short-term pH regulation depended linearly on the a transient alkalization (Felle and Bertl, 1986; Steigner pH-deviation with no indication of a feedback control et al., 1988;Okazaki et al., 1994).These pH deviations loop.Vacuolar light-dependent pH changes were were shown to be caused by the light-dependent proton measured to investigate further which pH-stat mechtranslocation across the thylakoid membrane.Proton anisms are responsible for the back regulation of the fluxes across the chloroplast envelope propagate the pH light-dependent cytosolic pH changes.Vacuolar pHdeviations from the chloroplast stroma to the cytosol changes were too small to explain the recovery of the ( Thaler et al., 1992;Hansen et al., 1993).cytosolic pH after darkening or illumination by H+The transient character of these light-dependent pH fluxes across the tonoplast.changes is not understood.It is generally assumed that efficient short-term pH-control mechanisms are respons-Key words: Eremosphaera viridis, ion-selective microible for the reversal of the light-dependent pH changes electrodes, light-induced pH-changes, pH-regulation, (Guern et al., 1991), but no experimental evidence exists vacuolar pH.and nothing is known about the nature of the pH-stat mechanisms involved.