Ion determinations within Azolla leaf cavities by microelectrodes

Calcium, potassium, sodium and chloride microelectrodes directly assembled were used for the in situ measurements of the concentrations of various ions within the Azolla leaf cavities. Moreover, pH was measured using a micro pH electrode. The results showed that: apical leaf cavities (apex and group I) contained 2–4 × 10−5 M calcium, 1.5 × 10−4 M sodium, 3 × 10−3 M potassium, 0.45 M chloride and had pH 6.0; median leaf cavities (groups II and III) which evidenced the highest nitrogenase activity, showed 1.5 × 10−4 M calcium, 2.5–3.5 × 10−4 M sodium, 1–2 × 10−3 M potassium, 0.45-0.35 M chloride and pH 6.5; basal leaf cavities (group IV), in which nitrogenase activity declined, contained 3 × 10−3 M calcium, 1 × 10−3 M sodium, 4 × 10−3 M potassium, 0.25 M chloride and had pH 5.7. These data suggest an active utilization of calcium, sodium and potassium in median leaf cavities from the Azolla-Anabaena system that could promote the high nitrogenase activity of the cyanobacterium. In basal leaf cavities, the increase of the various ion concentrations could be due to a decreased ability in their uptake from the association