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Articles

Polar Calcium Flux in Sunflower Hypocotyl Segments ¹

I. The Effect of Auxin

Department of Biological Sciences, Kent State University, Kent, Ohio 44242

The flux of Ca²⁺ at the apical or basal ends of short sunflower (Helianthus annuus L.) hypocotyl segments was monitored using a Ca²⁺-specific electrode. A higher Ca²⁺ efflux was observed at the apical end relative to the basal end, indicating a net polar flux of Ca²⁺. The extreme low mobility of Ca²⁺ in the isolated segment makes it likely that the observed Ca²⁺ fluxes are of localized origin, that is, from the parenchyma cells close to the exposed cut ends and may represent acropetal transport of Ca²⁺ at the cellular level. The rate of Ca²⁺ efflux depended on the concentration of Ca in the seedling medium. Incubation of hypocotyl segments in 10 mM CaCl₂ for 24 h did not eliminate the net acropetal flux of Ca²⁺ at the apical end.

IAA, as well as the synthetic auxin -naphthaleneacetic acid, significantly enhanced Ca²⁺ efflux; the non-auxin analog, -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 Ca²⁺ efflux since the presence of the auxin transport inhibitor 2,3,5-triiodobenzoic acid eliminated the auxin-promoted Ca²⁺ efflux. A model for how auxin promotion of Ca²⁺ 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.