Cobalt and Plant Development

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Abstract
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.