Passive Proton Conductance Is the Major Reason for Membrane Depolarization and Conductance Increase in Chara buckellii in High-Salt Conditions

Salt tolerance at single cell level in giant-celled Characeae

Characean plants provide an excellent experimental system for electrophysiology and physiology due to: (i) very large cell size, (ii) position on phylogenetic tree near the origin of land plants and (iii) continuous spectrum from very salt sensitive to very salt tolerant species. A range of experimental techniques is described, some unique to characean plants. Application of these methods provided electrical characteristics of membrane transporters, which dominate the membrane conductance under different outside conditions. With this considerable background knowledge the electrophysiology of salt sensitive and salt tolerant genera can be compared under salt and/or osmotic stress. Both salt tolerant and salt sensitive Characeae show a rise in membrane conductance and simultaneous increase in Na(+) influx upon exposure to saline medium. Salt tolerant Chara longifolia and Lamprothamnium sp. exhibit proton pump stimulation upon both turgor decrease and salinity increase, allowing the membrane PD to remain negative. The turgor is regulated through the inward K(+) rectifier and 2H(+)/Cl(-) symporter. Lamprothamnium plants can survive in hypersaline media up to twice seawater strength and withstand large sudden changes in salinity. Salt sensitive C. australis succumbs to 50-100 mM NaCl in few days. Cells exhibit no pump stimulation upon turgor decrease and at best transient pump stimulation upon salinity increase. Turgor is not regulated. The membrane PD exhibits characteristic noise upon exposure to salinity. Depolarization of membrane PD to excitation threshold sets off trains of action potentials, leading to further loses of K(+) and Cl(-). In final stages of salt damage the H(+)/OH(-) channels are thought to become the dominant transporter, dissipating the proton gradient and bringing the cell PD close to 0. The differences in transporter electrophysiology and their synergy under osmotic and/or saline stress in salt sensitive and salt tolerant characean cells are discussed in detail.

Membrane potential fluctuations in Chara australis: a characteristic signature of high external sodium

We have studied fluctuations in membrane PD in Chara australis at frequencies between 1 and 500 mHz, by classical noise analysis and by inspection of the PD time-course. The former shows (1) a quasi-Lorentzian (1/f (2)) rise of noise power as frequency falls, and (2) a marked increase in noise power when the cell is exposed to high salinity (Chara australis is a salt-sensitive species). The latter shows that, as well as initiating depolarization, exposure to 50 mM Na as either chloride or sulfate usually initiates a continuous but random series of small depolarizations which gives rise to the increase in noise and whose mechanism is discussed.

Effect of a Single Excitation Stimulus on Photosynthetic Activity and Light-dependent pH Banding in Chara Cells

Using pH microelectrodes and a Microscopy PAM (pulse-amplitude modulated) chlorophyll fluorometer, it is shown that a propagation of an action potential in Chara corallina leads to transient suppression of spatially periodic pH profiles along the illuminated cell. The suppression was manifested as a large pH decrease in the alkaline zones and a slight pH increase in the acid zones. The propagating action potential diminished the maximum yield of chlorophyll fluorescence (F(m)') in the alkaline cell regions, as well as the quantum yield of photosystem II photochemistry, without affecting F(m)' in the acid cell regions. The results indicate an interference of membrane excitation in the mechanisms responsible for pH banding patterns in Characean algae. Apparently, the electrical excitation of the plasma membrane in the alkaline cell regions initiates a pathway that can modulate membrane events at the thylakoid membrane.

Plasma Membrane Na+ Transport in a Salt-Tolerant Charophyte (Isotopic Fluxes, Electrophysiology, and Thermodynamics in Plants Adapted to Saltwater and Freshwater)

In salt-tolerant Chara longifolia, enhanced Na+ efflux plays an important role in maintaining low cytoplasmic Na+. When it is cultured in fresh water (FW), C. longifolia has a higher Na+ efflux than the obligate FW Chara corallina, although pH dependence and inhibitor profiles are similar for both species (J. Whittington and M.A. Bisson [1994] J Exp Bot 45: 657-665). When it is cultured in saltwater, C. longifolia has a Na+ efflux of 264 [plus or minus] 14 nmol m-2 s-1 at pH 7, 13 times higher than FW-adapted cultures and 31 times higher than C. corallina. As in FW-adapted plants, efflux is highest at pH 5, but pH dependence is less steep and more linear in cells adapted to saltwater. In plants of both species from FW cultures, Na+ efflux is inhibited by Li+ at pH 5 but not at pH 7 or 9, whereas in the salt-adapted C. longifolia, Li+ inhibits Na+ efflux at pH 7 and 9 but not at pH 5. Amiloride inhibits Na+ efflux in salt-adapted cells but not in FW cells. We conclude that a new type of Na+ efflux system is induced in salt-adapted plants, although both systems have characteristics suggestive of a Na+/H+ antiport. In all cases, a 1:1 Na+/H+ antiport would have sufficient energy to maintain the cytoplasmic Na+ activities measured at pH 5 and 7 but not at pH 9, which suggests that another efflux system must be operating at pH 9.