Short‐term regulation of cytosolic Ca2+, cytosolic pH and vacuolar pH under NaCl stress in the charophyte alga Nitellopsis obtusa

Abstract

ABSTRACTIsolated characean internodal cells of Nitellopsis obtusa can be stored in artificial pond water for many days, but they cannot survive in 100mol m−3 NaCl solution unless more than several mol m−3 Ca2+ is added. Short‐term effects of NaCl stress on the cytosolic concentration of Ca2+ ([Ca2+]c), cytosolic pH (pHc) and vacuolar pH (pHv) were studied in relation to the external concentration of Ca2+ ([Ca2+]e). Changes in [Ca2+]c were measured with light emission from a Ca2+‐sensitive photoprotein, semisynthetic fch‐aequorin which had been injected into the cytosol. Both pHc and pHv were measured with double‐barrelled pH‐sensitive microelectrodes. When internodal cells were treated with 100 mol m−3 NaCl (0–1 mol m−3 NaCl (0.1 mol m−3 [Ca2+]e), [Ca2+]c increased and then recovered to the original level within 60 min. The time course of the transient change in [Ca2+]c was not influenced by the level of [Ca2+]c (0.1 and 10 mol m−3). In some cases, the transient increase in [Ca2+]c was induced only by increasing external osmotic pressure with sorbitol. In response to treatment with 100 mol m−3 NaCl (0.1 mol m−3 [Ca2+]c), pHc decreased by 0.1–0.2 units after 10min but recovered after 30–60 min, while pHv increased by 0.4–0.5 units after 2–50 min and tended to recover after 60 min. The initial changes in both pHc and pHv were suppressed when [Ca2+]e was raised from 0.1 to 10mol m−3. These results show that the charophyte alga Nitellopsis can regulate [Ca2+]c, pHc and pHv under NaCl stress in the short term and that the protective effect of Ca2+ on salinity stress is apparently unrelated to perturbation of Ca2+ and pH homeostasis.