Current-voltage (I/V) analysis and pharmacological dissection were applied to membranes of iMinprothamnium at the time of hypotonic stress. At least three types of process were found to be involved in the response to this stress.(1) The first lOmin of exposure to hypotonic medium resulted in a depolarization of about 50 mV accompanied by a decrease or no change in conductance. This depolarization occurred with either K"^ or Ca'^* (and consequently CI") channels inactivated.(2) The Cr channels opened mainly in the first 15niin of the hypotonic stress, increasing the memhrane conductance hy about an order of magnitude.(3) The K* conductance rose as the CI" conductance started to diminish and reached a maximum after ahout 40 min.Both types of channel were strongly potential-dependent with a conductance peak between -150 and OmV. An inactivation of K*or Cl~ channels resulted tn moving the memhrane potential away from the conductance maximum toward either /i^ or E^i, diminishing the ion efflux (and turgor regulation). The time courses of the conductance increases remained the same, suggesting that the conductance changes are not driven hy feedback to some preset turgor level. The electrophysiology of the Lamprothamnium transporters is compared to that of salt-sensitive charophytes.

This paper investigates the impact of increased salinity on touch-induced receptor and action potentials of]ext was high, the background conductance increased to a lesser extent and proton pump currents were stimulated, establishing a PD narrowly negative to APthreshold. Cells did not spontaneously fire, but became hypersensitive to touch. Even slight touch stimulus induced an action potential and further repetitive firing. The duration of each excitation was extended when [Ca 2+ ]ext was low. Cell viability was prolonged in the absence of touch stimulus. Chara cells eventually depolarize and die in the saline media, but touch-stimulated and spontaneous excitation accelerates the process in a Ca 2+ -dependent manner. Our results have broad implications for understanding the interactions between mechano-perception and salinity stress in plants.

Lamprothamnium is a salt-tolerant charophyte that inhabits a broad range of saline environments. The electrical characteristics of Lamprothamnium cell membranes were modeled in environments of different salinity: full seawater (SW), 0.5 SW, 0.4 SW, and 0.2 SW. The cells were voltage-clamped to obtain the I/V (current-voltage) and G/V (conductance-voltage) profiles of the cell membranes. Cells growing at the different salinities exhibited one of three types of I/V profiles (states): pump-, background- and K(+)-states. This study concentrates on the pump- and background-states. Curved (pump-dominated) I/V characteristics were found in cells with resting membrane PDs (potential differences) of -219 +/- 12 mV (in 0.2 SW: 6 cells, 16 profiles), -161 +/- 12 mV (in 0.4 SW: 6 cells, 7 profiles), -151 +/- 12 mV (in 0.5 SW: 6 cells, 12 profiles) and -137 +/- 12 mV (in full SW: 8 cells, 13 profiles). The linear I/V characteristics of the background-state were found in cells with resting PDs of -107 +/- 12 mV (in 0.4 SW: 7 cells, 12 profiles), -108 +/- 12 mV (in 0.5 SW: 7 cells, 10 profiles) and -104 +/- 12 mV (in full SW: 3 cells, 5 profiles). The resting conductance (G) of the cells progressively increased with salinity, from 0.5 S x m(-2) (in 0.2 SW) to 22.0 S x m(-2) (in full SW). The pump peak conductance only rose from 2 S x m(-2) (0.2 SW) to 5 S x m(-2) (full SW), accounting for the increasingly depolarized resting PD observed in cells in more saline media. Upon exposure to hypertonic medium, both the pump and an inward K+ rectifier were stimulated. The modeling of the I/V profiles identified the inward K+ rectifier as an early electrical response to hypertonic challenge.

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