EFFECT OF CALCIUM-CHANNEL BLOCKERS ON THE IAA-INDUCED CELL ELONGATIONOF SUNFLOWER HYPOCOTYL SEGMENTS

Ca2+-channel blockers, verapamil and diltiazem, strongly inhibited IAA -induced elongation of sunflower (Helianthus annuus L.) hypocotyl segm ents. Pretreatment with Ca2+-channel blockers (0.1 mM verapamil or dil tiazem) inhibited the initiation of IAA-induced elongation, but not fu sicoccin-induced elongation. Verapamil, when applied 60 min after IAA, began to reduce the steady-state elongation rate with a short lag tim e of ca. 10 min. This inhibition of elongation by Ca2+-channel blocker s was antagonized by Ca2+ pretreatment-even at low concentration (less than 50 mu M). Cations such as Mn2+, Mg K+, and Na+ also showed the a ntagonism with respect: to the inhibitory effect of the blockers, wher eby the effect of divalent cations was much more effective than monova lent cations. The antagonism of Ca2+ against the inhibitory effect of the blockers occurred optimally ata specific Ca2+ concentration. These results suggest that Ca2+ and Ca2+-channel blockers competitively int eract against the Ca2+ channel and that calcium flux through Ca2+ chan nels plays an important role in auxin-induced cell elongation.