ISSN:
1365-3040
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
In isolated Elodea densa leaves, the relationships between H+ extrusion (-ΔH+), K+ fluxes and membrane potential (Em) were investigated for two different conditions of activation of the ATP-dependent H+ pump. The ‘basal condition’ (darkness, no pump activator present) was characterized by low values of-ΔH+ and K+ uptake (ΔK+), wide variability of the −ΔH+/ΔK+ ratio, relatively low membrane polarization and Em values more positive than EK for external K+ concentrations (|K+]o of up to 2mol m−3. A net K+ uptake was seen already at [K+]o below 1 mol m−3, suggesting that K+ influx in this condition was a thermodynamically uphill process involving an active mechanism. When the H+ pump was stimulated by fusicoccin (FC), by cytosol acidification, or by light (the ‘high polarization condition’), K+ influx largely dominated K+ and C− efflux, and the −ΔH+/ΔK+ ratio approached unity. In the range 50 mmol m−3−5 mol m−3 [K+]0, Em was consistently more negative than EK. The curve of K+ influx at [K+]0 ranging from 50 to 5000mmol m−3 fitted a monophasic, hyperbolic curve, with an apparent half saturation value = 0–2 mol m−3. Increasing |K+]0 progressively depolarized Em, counteracting the strong hyperpolarizing effect of FC. The effects of K+ in depolarizing Em were well correlated with the effects on both K+ influx and −ΔH+, suggesting a cause-effect chain: K+0 influx → depolarization → activation of H+ extrusion. Cs+ competitively inhibited K+ influx much more strongly in the ‘high polarization’ than in the ‘basal’ condition (50% inhibition at [Cs+]/[K+]0 ratios of 1:14 and 1:2, respectively) thus confirming the involvement of different K+ uptake systems in the two conditions. These results suggest that in E. densa leaves two distinct modes of interactions rule the relationships between H+ pump, membrane polarization and K+ transport. At low membrane polarization, corresponding to a low state of activation of the PM H+-ATPase and to Em values more positive than EK, K+ influx would mainly
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-3040.1995.tb00607.x
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