CONTROL OF SHOOT‐RHIZOME DIMORPHISM IN THE WOODY MONOCOTYLEDON, CORDYLINE (AGAVACEAE)

In Cordyline terminalis negatively geotropic leafy shoots and positively geotropic rhizomes develop from single axillary buds on either shoots or rhizomes. All axillary buds have similar morphogenetic potential when released from apical dominance. Experiments in which the orientation of the apex is changed, organs removed, or growth regulators applied indicate that after a rhizome is initiated, it is maintained as a rhizome by auxin originating in the leafy shoot. When auxin levels are lowered by changes in the orientation of the axis or shoot removal, the rhizome apex becomes a shoot apex, which appears to be the stable state of the actively growing apex. Benzyl adenine when applied exogenously to the apex or lateral buds has the same effect as lowering the auxin level. Gibberellic acid has no effect on the apex or lateral buds. High levels of exogenous naphthaleneacetic acid cause bud release and development of rhizomes from previously inhibited axillary buds of the shoot. However, it was not possible to convert a shoot apex into a rhizome apex by auxin treatment. It is suggested that the release of buds on the lower side of horizontal branches and of buds directly above a stem girdle is caused by high auxin levels on the lower side or distal to the girdle. The experimental results are discussed in relation to naturally occurring shoot-rhizome dimorphism. THE AGAVACEAE (sensu Hutchinson, 1959) includes many subtropical and tropical monocotyledons which are woody and often arborescent and which have secondary growth and the ability of branches to grow out. and establish secondary vascular continuity with the parent stem (Tomlinson and Zimmermann, 1969; Tomlinson and Fisher, 1971). Although this group is exceptional for the monocotyledons, the morphology of a representative species, Cordyline terminalis L., has been described in detail only recently (Fisher and Tomlinson, 1972). In addition to possessing secondary growth, C. terminalis also produces positively geotropic rhizomes from axillary buds (Fig. 1, F-K), starting with the seedling (Fig. 1, C) in which the lower portion of the original seedling axis is displaced by the seedling rhizome. When released from apical inhibition, axillary buds usually produce negatively geotropic leafy shoots (Fig. 1, A, B, and D). Field-grown plants are particularly striking since many rhizomes are aerial, often being initiated several meters above the soil (Fig. 1, I). The full dimorphic expression which the axillary bud can produce is shown diagrammatically in Fig. 1. From a morphogenetic ' Received for publication 3 March 1972. This work was supported in part by a postdoctoral fellowship from the Cabot Foundation, Harvard University and by National Science Foundation grant GB5762X awarded to P. B. Tomlinson. The encouragement and helpful criticism of Dr. Tomlinson is gratefully acknowledged. All drawings are by Priscilla Fawcett, botanical illustrator for Fairchild Tropical Garden. The author thanks the proprietor of Shaw's Nursery, South Miami, for donating plant material, and Dr. M. J. Jaffe for his critical reading of the manuscript. point of view, it should be noted that although leafy shoots and rhizomes develop from similar axillary buds, only the rhizome apex can change directly into a leafy shoot (Fig. 1, K). The reverse change of the apical meristem of the leafy shoot into a rhizome has never been observed. This paper presents the results of experiments carried out to determine the factors which are required for both initiation and maintenance of rhizome growth and factors which release the inhibition of axillary buds. Many of these experiments are elementary, but this is necessary because this group of plants has been studied experimentally only once (Sachs, 1882) and then relatively superficially. The lack of previous studies is a commentary on the general state of botanical knowledge of the developmental physiology and morphology of plants that are woody and predominantly tropical. MATERIALS AND METHODS-Mature field grown plants (4-5 m tall) consisting of several related cultivars were grown at Shaw's Nursery, South Miami, Florida. Cuttings of vigorous leafy shoots and aerial rhizomes, as well as seeds taken from these plants, were grown in a mixture of half perlite-half soil under greenhouse conditions. Alpha-naphthaleneacetic acid (NAA), N-6benzyladenine (BA), and gibberellic acid (GA) were dispersed in anhydrous lanolin on a weightweight basis after first being dissolved in 1-2 ml of 95 % ethanol. An equal amount of 95 % ethanol was added to the control lanolin paste. Very high concentrations of regulators, especially BA, were necessary for physiological responses.