Polar Calcium Flux in Sunflower Hypocotyl Segments

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Abstract
Calcium flux in sunflower (Helianthus annuus L. cv Russian mammoth) hypocotyl was measured with a Ca2+ electrode as the increase or decrease in Ca2+ in an aqueous solution (10 micromolar CaCl2) in contact with either the basal or apical end of 20 millimeter segments. Ca2+ efflux was significantly higher at the apical end compared with the basal end; this apparent polarity was maintained even when the segments were inverted. No significant difference was observed in the cation exchange capacity of apical and basal cell walls that could explain the difference in Ca2+ efflux at opposite ends of the hypocotyl segment. The presence of exogenous indoleacetic acid (IAA) in the segment medium resulted in the promotion of both Ca2+ efflux and segment elongation. However, osmotic inhibition of the IAA-induced elongation did not result in inhibiting the IAA-induced Ca2+ efflux. Ca2+ efflux was inhibited by cyanide. Lowering the temperature from 25°C also caused the gradual reduction of Ca2+ efflux; at 5°C the hypocotyl segments showed a net absorption of Ca2+ from the segment medium. These findings support the suggestion that: (a) the observed Ca2+ efflux in hypocotyl segments is probably the manifestation of the system which maintains the transmembrane Ca2+ gradient at the cellular level. (b) The acropetal polarity of Ca2+ efflux may be the result of the involvement of Ca2+ in the basipetal transport of IAA.