Ionic currents flow through Micrasterias and Closterium cells during expansion of the primary cell wall

The results of studies of Micrasterias rotata (Grev.) Ralfs, M. thomasiana Archer (biradiate and uniradiate forms) and Closterium sp. using one- and two-dimensional vibrating probes show that transcellular ionic currents are detectable only around cells undergoing expansion of the primary cell wall (half-cell); current enters local regions of expansion and exits over both the rigid surface of the secondary wall and regions of the primary wall where hardening of the wall prevents further expansion. Current densities remain at steady levels until expansion stops with maturation of the primary wall, whereupon currents are no longer detectable. The temporal and spatial correlation between the currents and regions of wall expansion is particularly evident because morphogenesis of the half-cell is a determinate process. Measurements of inward currents ranged from 0.1 to 5.4 μA · cm(-2), and outward currents ranged from-0.05 to -1.5 μA · cm(-2) measured at 18 μ from the cell surface. The results of ion substitution and channel-blocker studies indicate that the currents may be carried at least in part by Ca(2+), Cl(-), H(+) and K(+) ions. The possible role of a Ca(2+) influx during tip growth in desmids is discussed.