Polar Calcium Flux in Sunflower Hypocotyl Segments

The flux of Ca(2+) at the apical or basal ends of short sunflower (Helianthus annuus L.) hypocotyl segments was monitored using a Ca(2+)-specific electrode. A higher Ca(2+) efflux was observed at the apical end relative to the basal end, indicating a net polar flux of Ca(2+). The extreme low mobility of Ca(2+) in the isolated segment makes it likely that the observed Ca(2+) fluxes are of localized origin, that is, from the parenchyma cells close to the exposed cut ends and may represent acropetal transport of Ca(2+) at the cellular level. The rate of Ca(2+) efflux depended on the concentration of Ca in the seedling medium. Incubation of hypocotyl segments in 10 mm CaCl(2) for 24 h did not eliminate the net acropetal flux of Ca(2+) at the apical end.IAA, as well as the synthetic auxin alpha-naphthaleneacetic acid, significantly enhanced Ca(2+) efflux; the non-auxin analog, beta-naphthaleneacetic acid, was ineffective. The transport of auxin, not merely its presence in the medium, was found to be a requisite for the enhancement of Ca(2+) efflux since the presence of the auxin transport inhibitor 2,3,5-triiodobenzoic acid eliminated the auxin-promoted Ca(2+) efflux. A model for how auxin promotion of Ca(2+) efflux could play a role in promoting subsequent auxin secretion is proposed. Calcium probably serves as a ;second messenger', as it does in the secretion of various substances by animal cells.