Calcium influx at the plasmalemma of isolated guard cells of Commelina communis

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Abstract
The influx of 45Ca into isolated guard cells of Commelina communis L. has been measured, using short uptake times, and washing in ice-cold La3+-containing solutions to remove extracellular tracer after the loading period. Over 0.5–4 min the uptake was linear with time, through the origin. Over 20–200μM external Ca2+ the influx measured with 10–20 mM external KCl was in the range 0.3–2.3 pmol·cm-2·s-1 (on the basis of estimated guard-cell area); with only 1 mM KCl externally the 45Ca influx was significantly reduced, in the range 0.3–1.1 pmol·cm-2·s-1 for external Ca2+ of 50–100 μM. The results indicate that the Ca-channel is voltage-sensitive, opening with depolarisation. No consistent effect of the addition of abscisic acid could be found. In different experiments, on the addition of 0.1 mM abscisic acid the Ca2+ influx was sometimes stimulated by 28–79%, was sometimes unaffected, and was sometimes inhibited by 16–29%. The results rule out a long-lasting stimulation of 45Ca influx by ABA, but they do not rule out a transient stimulation followed by inhibition, perphaps as a consequence of down-regulation of Ca2+ influx by increasing cytoplasmic Ca2+. The hypothesis that ABA may act via an action on Ca2+ influx, increasing cytoplasmic Ca2+, with consequent effects on voltage-dependent and Ca2+-dependent ion channels in both plasmalemma and tonoplast, is neither proved nor disproved by these results.