SummaryReactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K + channel responsible for K + efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca 2+ , tetraethylammonium, Ba 2+ , Cs + and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K + channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K + efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K + -channel activation. In animals, activation of K + -efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K + -channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K + channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K + channels.

Electrolyte leakage accompanies plant response to stresses, such as salinity, pathogen attack, drought, heavy metals, hyperthermia, and hypothermia; however, the mechanism and physiological role of this phenomenon have only recently been clarified. Accumulating evidence shows that electrolyte leakage is mainly related to K(+) efflux from plant cells, which is mediated by plasma membrane cation conductances. Recent studies have demonstrated that these conductances include components with different kinetics of activation and cation selectivity. Most probably they are encoded by GORK, SKOR, and annexin genes. Hypothetically, cyclic nucleotide-gated channels and ionotropic glutamate receptors can also be involved. The stress-induced electrolyte leakage is usually accompanied by accumulation of reactive oxygen species (ROS) and often results in programmed cell death (PCD). Recent data strongly suggest that these reactions are linked to each other. ROS have been shown to activate GORK, SKOR, and annexins. ROS-activated K(+) efflux through GORK channels results in dramatic K(+) loss from plant cells, which stimulates proteases and endonucleases, and promotes PCD. This mechanism is likely to trigger plant PCD under severe stress. However, in moderate stress conditions, K(+) efflux could play an essential role as a 'metabolic switch' in anabolic reactions, stimulating catabolic processes and saving 'metabolic' energy for adaptation and repair needs.

Summary. Cation channels of passive transport in the plasmalemma of Nitella flexilis cells at rest were studied by the voltageclamp technique using microelectrodes. Two types of potassium channels have been identified. They are activated at different voltages: over -100 to -80 mV (D-channels) and below -100 mV (H-channels). The zero-current potential of instantaneous voltage-current curves (IVCC's) for both types of channels shifts by 50 to 55 mV in response to a 10-fold increase of K + concentration in the solution. Ion movement in D-channels follows the free diffusion mechanism; in H-channels the independence principle is violated. The channel selectivity (in the order of decreasing permeability) is: K + > Rb + > NH~ > Na + > Li + > Cs + > TEA + choline*. It has been found that D-channel Cs + block is potential dependent while tetraethylammonium (TEA +) blocks Hchannels in a potential-independent manner, but H + ions do not affect the inward potassium current of the channels. Two types of potassium channels appear to be located in different parts of the membrane and their entrance parts are of different structure.

Cuz+ is one of the most important micronutrients in plants. The normal growth and development of plants requires adequate Cu2+. However, the range of Cu2+ concentrations that have an appropriate effect on plants is very narrow; even a slight elevation of the upper leve1 of physiologically allowed Cu2+ concentrations leads to toxic effects in the cell resulting from disturbances in ionic homeostasis and metabolism (Loneragan et al., 1981; Robinson, 1983; Bergmann, 1992). At present the treatment of plants with Cu2+-containing fungicides remains one of the most widespread approaches for cultivating tobacco, grape, tomato, cotton, hops, etc., although it sometimes leads to plant injury and serious losses in both the quantity and quality of the yield (Bergmann, 1992).Cu2+ toxicity differs even in related species (Macnair, 1992;Singh et al., 1994). Except for some tolerant species, the optimum Cu2+ concentration for higher plants is in the range of 1 to 100 p~ (Bergmann, 1992).

Effects of Cu2+ on a non-specific conductance and H+-ATPase activity in the plasma membrane of the freshwater alga Nitella flexilis L. Agardh was studied using a conventional microelectrode voltage-clamp technique. We show that a Cu2+-induced increase in the non-specific conductance is related to the formation of pores in the plasma membrane. Pore formation is the result of unidentified chemical reactions, since the Q10 for the rate of increase of conductance over time was about 3. Various oxidants and antioxidants (10 mmol/l H2O2, 10 mmol/l ascorbate, 100 microg/ml superoxide dismutase, and 100 microg/ml catalase) did not alter Cu2+-induced changes in the plasma membrane conductance, suggesting that the effect of Cu2+ was unrelated to peroxidation of plasma-membrane lipids. In contrast, organic and inorganic Ca2+-channel antagonists (nifedipine, Zn2+, Cd2+, Fe2+, Ni2+) inhibited the Cu2+-induced non-specific conductance increase. This suggests that changes in Ca2+ influx underlie this effect of Cu2+. Decreasing the pH or the ionic strength of external solutions also inhibited the Cu2+-induced plasma-membrane conductance increase. Copper was also found to inhibit plasma-membrane H+-ATPase activity with half-maximal inhibition occurring at about 5-20 micromol/l and full inhibition at about 100-300 micromol/l. The Hill coefficient of Cu2+ inhibition of the H+-ATPase was close to two.