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 involvement in adventitious root initiation from the hypocotyl explants of sunflower (Helianthus annuus L.). When hypocotyl explants were cultured on MS medium (containing calcium), 1 mg l-1 IAA was found to be optimal for root induction. However, the hypocotyl explants washed in EGTA (10-5M) solution for the removal of extracellular calcium, when cultured on medium containing IAA and calcium, exhibited enhanced rooting response. When EGTA-washed explants were cultured on the medium supplemented with lanthanum chloride (10-6 and 10-5M), it resulted in the inhibition of the rooting response and this inhibitory effect could be alleviated by the simultaneous addition of IAA. Similar observations 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 discussed in light of initial lack of polarity in the decapitated hypocotyl segments subjected to auxin treatment.