Effects of salt-adaptation and salt-stress on extracellular acidification and microsome phosphohydrolase activities in tomato cell suspensions

The effects of NaCl-adaptation and NaCl-stress on in vivo H+ extrusion and microsomal vanadate- and bafilomycin-sensitive ATPase and PPase activities were studied in tomato cell suspensions. Acidification of the external medi um by 50 mM NaCl-adapted and non-adapted (control) tomato cells was similar . Extracellular acidification by both types of cells during the first hour of incubation with 2 muM fusicoccin (FC) in the presence of 100 mM NaCl was lightly increased while in the presence of 100 mM KCl it was increased by 3 (control)- and 6.5 (adapted)-fold. Extracellular alkalinization after 2 h of cell incubation in 100 mM NaCl indicated the possibility that a Na+/Hexchange activity could be operating in both types of cells. Moreover, acid ification induced by adding 100 mM NaCl + FC to non-adapted cells was relat ively less affected by vanadate than that induced by 5 mM KCl + FC, which s uggested that salt stress could induce some component other than H+ extrusi on by H+-ATPase. In addition, no differences were observed in microsomal va nadate-sensitive ATPase activity among control, NaCl-adapted and NaCl-stres sed cells, while K+-stimulated H+-PPase and bafilomycin-sensitive H+-ATPase activities were higher in microsomes from NaCl-adapted than in those from control cells. Likewise, the stimulation of in vivo H+ extrusion in NaCl ad apted cells under NaCl or KCl stress in the presence of FC occurred with an inhibition of H+-PPase and bafilomycin-sensitive H+-ATPase activities and without changes in the vanadate-sensitive H+-ATPase activity. These results suggest that the stimulation of tonoplast proton pumps in NaCl-adapted cel ls, without changes in plasmalemma H+-ATPase, could serve to energize Na+ e fflux across the plasmalemma and Na+ fluxes into vacuoles catalyzed by the Na+/H+ antiports.