Acetabularia

Abstract: With chlorotetracycline (CTC)-fluorescence a tip-to-base Ca2+ gradient is visualized in all tested, tip-growing plant cells: pollen tubes of Lilium longiflorum, root hairs of Lepidium sativum, moss caulonema of Funaria hygrometrica, fungal hyphae of Achlya and in the alga Acetabularia mediterranea. The fluorescence gradients in the different species vary in intensity and extension. Sometimes a punctate mobile CTC-fluorescence, in the size range of mitochondria, is observed. Bursting cells lose their fluorescence rapidly, indicating a cytoplasmic localization of the gradient. Only in Acetabularia is the wall also fluorescent with CTC. The results are interpreted as evidence for a general role of a calcium gradient in tip growth.

Abstract: The cytoplasm of an Acetabularia cell is normally at a potential of about -170 mv relative to the external solution; the vacuole is also at this potential. Although there is strict flux equilibrium for all ions, the potential is more negative than the Nernst potentials of any of the permeating ions. Darkness, CCCP, low temperature, and reducing [Cl-]o by a factor of 25 all rapidly depolarize the membrane and inhibit Cl- influx. Some of these treatments do not inhibit the effluxes of K+ and Na+. Increasing [K+]o also depolarizes the membrane both under normal conditions and at low temperature; in the latter case the membrane is partially depolarized in normal seawater (low [K+]o) and in high [K+]o positive potentials of up to +15 mv are attained. It is concluded that the membrane potential is controlled by the electrogenic influx of Cl-, and also, at least in some circumstances, by the diffusion of K+. In addition, it is suggested that electrogenic efflux of H+ may be important in transient nonequilibrium situations. An Appendix deals with the interpretation of simple nonsteady-state tracer kinetic data.