Cobalt and plant development: interactions with ethylene in hypocotyl growth

Co²⁺ promoted elongation of hypocotyl segments of light-grown cucumber (Cucumis sativus) seedlings. Time course and dose response data are presented and interactions with IAA, gibberellin, cyclohexanol, and cotyledons described. Segments without cotyledons responded to Co²⁺ only if grown in gas-tight vessels with IAA added. When bases of cotyledons were ringed with an inhibitor of auxin transport, Co²⁺ caused no growth promotion in the hypocotyl. Co²⁺ prevented lateral swelling of hypocotyls treated with supraoptimal IAA. Removal of ethylene from the atmosphere reduced the Co²⁺ response, but Co²⁺ did not counteract the inhibitory effect of increased ethylene levels. These results are consistent with the hypothesis that Co²⁺ promotes hypocotyl elongation by inhibiting ethylene production. The hypothesis was confirmed by a direct demonstration that Co²⁺, at growth-promoting concentrations, powerfully inhibited ethylene production in the cucumber hypocotyl.