ATP‐driven proton pumping in two species of Chara differing in salt tolerance

Chara corallina is an obligate freshwater alga, while C. buckellii can be grown in salt and freshwater culture. When grown in fresh water, C. buckellii has electrophysiological properties similar to C. corallina, but when cultured in salt water, it has a less negative membrane potential and has a higher conductance. We show in internally perfused, tonoplast‐free cells that the ATP‐dependence of the two species cultured in fresh water is similar, although C. buckellii hyperpolarizes at lower ATP concentrations. We determined the pump parameters in perfused and intact cells. Using both techniques, C. corallina and C. buckellii cultured in fresh water show similar values of E_p, G_p and I_p. However, there is a significant difference between the two techniques: E_p is more negative (–400 to –700 mV) in perfused cells than in intact cells (–220 to –260mV); G_p is lower (0·1–0·2 versus 0·3–0·9 S m⁻²); and I_p is higher (40–60 versus 10–18 mA m⁻²). Salt‐cultured C. buckellii was compared with freshwater C. buckellii using intact cells; G_p and E_p were similar, but I_p was much higher in salt‐cultured cells (60 versus 15mA m⁻²). This higher pump rate is due to the depolarization of the membrane of salt‐cultured algae, which is caused by a higher passive conductance. The significance of the less negative membrane potential and the higher rate of proton pumping is discussed with respect to the banding pattern and salt stress.