Involvement of calcium ion in turgor regulation upon hypotonic treatment in Lamprothamnium succinctum

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Abstract
When internodal cells of Lamprothamnium succinetum were exposed to a hypotonic medium containing more than 1 mol m−3 Ca2+, the elevated turgor pressure decreased and reached a steady state within 30–60 min. The hypotonic treatment caused the membrane potential to depolarize, with a time lag of ca. 1 min. The membrane conductance increased transiently with the same time lag and reached a peak value within 2–3 min. When the external Ca2+ concentration was lowered to 0.01 mol m−3, both turgor regulation and change in the membrane conductance were strongly inhibited, although the membrane depolarization was not affected. When the Ca2+ level was returned to the normal level, the cells recovered their ability for turgor regulation and the membrane conductance attained a peak value after ca. 15–30 s. This response time is definitely shorter than that needed for the conductance change in cells exposed to a hypotonic medium having a normal level of Ca2+ from the beginning. We thus conclude that at least two sequential processes are involved in turgor regulation: a Ca2+ ‐independent process, followed by a Ca2+‐dependent process.

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