PRIMARY PHLOEM REGENERATION WITHOUT CONCOMITANT XYLEM REGENERATION: ITS HORMONE CONTROL IN COLEUS

Phloem regeneration in the absence of xylem regeneration was evoked in number 5 internodes of Coleus blumei Benth. by severing xylemless phloem bundles. Its quantitative extent was estimated. To determine whether phloem regeneration is directly affected by auxin, or whether it is a secondary consequence of the auxin-dependent xylem regeneration which usually accompanies it, phloem regeneration was measured in decapitated plants from which auxin-producing leaves and buds had been removed (i.e., in “plant stumps”). In these stumps, 1% IAA in lanolin completely restored phloem regeneration to the intact plant level. In such stumps from which roots had been excised, and in excised internodes, IAA failed to restore it to this level. However, zeatin or zeatin riboside in aqueous solution applied to the bases of excised internodes receiving IAA at their apical ends restored phloem regeneration to the level of that in whole plants. When similarly tested, other cytokinins (kinetin, kinetin riboside, 2iP, and 2iPA), gibberellic acid (GA₃), glutamine, proline, sucrose, and a mixture of mineral salts failed to promote phloem regeneration. Glutamic acid, tested only once, was slightly promotive of it.