AUXIN-CALCIUM INTERACTION IN ADVENTITIOUS ROOT-FORMATION IN THE HYPOCOTYL EXPLANTS OF SUNFLOWER (HELIANTHUS-ANNUUS L.)

Auxin-calcium interaction has been studied to understand their involve ment in adventitious root initiation from the hypocotyl explants of su nflower (Helianthus annuus L.). When hypocotyl explants were cultured on MS medium (containing calcium), 1 mg l(-1) IAA was found to be opti mal for root induction. However, the hypocotyl explants washed in EGTA (10(-5)M) solution for the removal of extracellular calcium, when cul tured on medium containing IAA and calcium, exhibited enhanced rooting response. When EGTA-washed explants were cultured on the medium suppl emented with lanthanum chloride (10(-6) and 10(-5)M), it resulted in t he inhibition of the rooting response and this inhibitory effect could be alleviated by the simultaneous addition of IAA. Similar observatio ns have been made by using calcium channel blockers, verapamil and TMB -8, and also a calmodulin inhibitor, trifluoperazine. A net influx of extracellular calcium in the differentiating cells is thus presumed to accompany the auxin-induced response, These results have been discuss ed in light of initial lack of polarity in the decapitated hypocotyl s egments subjected to auxin treatment.