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We describe an anion channel in the plasmalemma of protoplasts isolated from wheat (Triticum aestivum L.) roots that is activated by aluminum (Al3+). In the whole-cell configuration, addition of 20-50 microM AlCl3 to the external solution depolarized the membrane and activated an inward current that could remain active for more than 60 min. The activation by Al3+ was rapid in 20% of protoplasts examined, whereas in another 30% a delay of more than 10 min occurred after Al3+ was added. Once the current was activated, changing the external Cl- concentration shifted the membrane reversal potential with ECl, showing that the channel is more selective for anions than cations (Ca2+, K+, tetraethylammonium+). The channel could be activated by Al3+, but not by La3+, and was observed in protoplasts isolated from the root apex but not in protoplasts isolated from mature root tissue. The anion channel antagonist niflumate inhibited the current in whole cell measurements by 83% at 100 microM. Outside-out patch recordings revealed a multistate channel with single-channel conductances of between 27 and 66 pS.

Short-term effect of oxygen-deficiency on the membrane potential difference (PD), membrane resistance of cortical cells and electrical coupling between cortical cells was investigated using excised wheat roots. Hypoxia rapidly depolarised the membrane potential of the cortical cells by about 60 mV, while hypoxia had little effect on the membrane resistance of the cells. No significant change in membrane resistance by potassium channel blockers, TEA+ and verapamil, under hypoxia was observed. The electrical coupling ratio, which is a measure of plasmodesmatal resistance, between cortical cells of wheat roots was 5.9 % in aerated solution and was not affected by the low oxygen treatment, suggesting that solute transport through cytoplasmic annulus of plasmodesmata could not be affected. The possible involvement of the endoplasmic reticulum in intercellular transport of solute and water is discussed.

The large conductance K+ channel in the tonoplast of Chara corallina has subconductance states (substates). We describe a method that detects substates by monitoring the time derivative of channel current. Substates near to the full conductance tend to have long durations and high probabilities, while those of smaller amplitude occur with less probability and short duration. The substate pattern is similar in cell-attached, inside-out and outside-out patches over a range of temperatures. The pattern changes at high Ca2+ concentration (10 mol m-3) on the cytoplasmic face of inside-out patches. One substate at approximately 50% of the full conductance is characterized by a high frequency of transitions from the full conductance level. This midstate conductance is not a constant proportion of the full conductance but changes as a function of membrane potential difference (p.d.) showing strong inward rectification. We suggest that the channel is a single pore that can change conformation and/or charge profile to give different conductances. The mean durations of the full conductance level and the midstate decrease as the membrane p.d. becomes more negative. Programs for analysis of channel kinetics based on an half-amplitude detection criterion are shown to be unsuitable for analysis of the K+ channel.

The characteristics of cation outward rectifier channels were studied in protoplasts from wheat root (Triticum aestivum L. and Triticum turgidum L.) cells using the patch clamp technique. The cation outward rectifier channels were voltage-dependent with a single channel conductance of 32 +/- 1 picosiemens in 100 millimolar KCl. Whole-cell currents were dominated by the activity of the cation outward rectifiers. The time- and voltage-dependence of these currents was accounted for by the summed behavior of individual channels recorded from outside-out detached patches. The K(+)/Na(+) permeability ratio of these channels was measured in a salt-sensitive and salt-tolerant genotype of wheat that differ in rates of Na(+) accumulation, using a voltage ramp protocol on protoplasts in the whole-cell configuration. Permeability ratios were calculated from shifts in reversal potentials following ion substitutions. There were no significant differences in the K(+)/Na(+) permeability ratios of these channels in root cells from either of the two genotypes tested. The permeability ratio for K(+)/Cl(-) was greater than 50:1. The K(+)/Na(+) permeability ratio averaged 30:1, which is two to four times more selective than the same type of channel in guard cells and suspension culture cells. Lowering the Ca(2+) concentration in the bath solution to 0.1 millimolar in the presence of 100 millimolar Na(+) had no significant effect on the K(+)/Na(+) permeability ratios of the channel. It seems unlikely that the mechanism of salt tolerance in wheat is based on differences in the K(+)/Na(+) selectivity of these channels.

The stationary volumetric elastic modulus (epsilon(s)) of the leaf cells of three seagrasses (Halophila ovalis (R.Br.) Hook, Zostera capricorni Aschers, and Posidonia australis Hook f.) was evaluated from estimates of epsilon(s) plus intracellular osmotic pressure (epsilon(s) + II(i)) and II(i). The estimates of (epsilon(s) + II(i)) were made using a linear displacement transducer to measure very small changes in thickness of leaf tissue produced by changes in external osmotic pressure (II(o)). epsilon(s) increases with increasing turgor pressure in each of the species and the maximum values of epsilon(s) are: 22 megapascals for H. ovalis, 17 megapascals for Z. capricorni, and 51 megapascals for P. australis.There is a hysteresis in thickness changes versus changes in II(o) which indicates a hysteresis in the relationship between volume and turgor pressure. The hysteresis results in epsilon(s) being different for swelling and for shrinking cells over the same range of II(o) and this may be important in other aspects of plant-water relations.A new design of an apparatus employing a linear displacement transducer for measuring very small changes in tissue thickness is described. The new design has the advantages of virtually frictionless movement and a precision of 0.05 micrometer.

The analysis of Sha'afi et al. (Sha'afi, Rich, Mickulecky, Solomon 1970 J Gen Physiol 55: 427-450) for determining solute permeability in red blood cells has been modified and applied to turgid plant cells. Following the addition of permeating solute to the external medium, a biphasic response of cell turgor can be measured with the pressure probe in isolated internodes of Chara corallina. After an initial decrease in turgor due to water flow (water phase), turgor increases due to the uptake of the solute (solute phase) until the original turgor is reattained. From the pressure/time course in the neighborhood of the minimum turgor, the permeability of the osmotic solute can be determined. The data obtained by the minimum method for rapidly permeating (ethanol, methanol) and slowly permeating (formamide, dimethylformamide) solutes are similar to those calculated from the half-time of pressure changes during the solute phase and to data obtained by other workers using radioactive tracers. The methods employing the pressure probe were applied to examine the effect of high pH (up to pH 11) on the membrane permeability. There appeared to be no effect of high pH on the permeability coefficients, reflection coefficients, and hydraulic conductivity.