Effects of magnesium, calcium and lanthanum ions on stomatal oscillations in Avena sativa L.

Mg(2+), Ca(2+) and La(3+) caused an increased period time of the transpiratory oscillations when given to excised oscillating Avena plants (plants without root system). The effect was reversible, i.e. after withdrawal of the ions the period time returned to its original value. In order to achieve the same period lengthening as with 2.5 mM La(3+), 20 mM Ca(2+) and 40 mM Mg(2+) was needed. The effects are discussed along two lines: (a) the ions interfere with ionic processes, central for the stomatal regulation, and (b) the ions decrease the water permeability of the guard cells. Simulations on a model, previously published in the literature, showed that an increased resistance against the water flow into the guard cells could explain the findings. The ions had no effect on the period length of the oscillations in intact plants, i.e. with root system.