A calcium influx precedes organogenesis in Graptopetalum

An account is given of an investigation of net ionic currents and specific ion fluxes occuring during the initiation of organogenesis in detached leaves of Graptopetalum paraguayense E. Walther, in which a dramatic change in growth polarity is cytomorphologically evident 3–5 d after leaf detachment from the plant. Using the vibrating probe, it was possible to identify a peak of ionic current which is focused over the area of the leaf base where organogenesis is initiated. This net current is largest during the initial 12h after leaf detachment. With ion‐selective microelectrodes capable of measuring H+, K+ and Ca2+ ion fluxes simultaneously in the same region of the leaf base, H+ and K+ fluxes remain relatively steady during the initial 24 h after detachment, while a large lanthanum‐sensitive Ca2+ influx decreases by 50% from 2 to 12h. By 24h, Ca2+ transport is dominated by an efflux. We present evidence from a quantitative comparison of the ion current data collected using these two techniques, that Ca2+, H+ and K+ transport accounts for the major electrogenic ion fluxes during 2 and 12 but not 24 h after leaf detachment. The possibility is addressed that these ion currents, which precede organogenesis, and in particular the predominant Ca2+ flux, play a role in the establishment of growth polarity in higher plant tissues.