Calcium influx at the plasmalemma of isolated guard cells of Commelina communis : Effects of abscisic acid.

The influx of (45)Ca into isolated guard cells of Commelina communis L. has been measured, using short uptake times, and washing in ice-cold La(3+)-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 Ca(2+) 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 (45)Ca influx was significantly reduced, in the range 0.3-1.1 pmol·cm(-2)·s(-1) for external Ca(2+) 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 Ca(2+) 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 (45)Ca influx by ABA, but they do not rule out a transient stimulation followed by inhibition, perphaps as a consequence of down-regulation of Ca(2+) influx by increasing cytoplasmic Ca(2+). The hypothesis that ABA may act via an action on Ca(2+) influx, increasing cytoplasmic Ca(2+), with consequent effects on voltage-dependent and Ca(2+)-dependent ion channels in both plasmalemma and tonoplast, is neither proved nor disproved by these results.