Stimulation of stem growth in vegetative tillers of Poa pratensis L. by ethephon: elongation of nodes

Application of ethephon ((2-chloroethyl)phosphonic acid, ethrel, CEPA) has been shown to stimulate elongation of internodes in vegetative tillers of Poa pratensis L. and Avena sativa L. (Van Andel 1970; Van Andel & Verkerke in press). Occasionally the plants produced one or two internodes of abnormal shape. This phenomenon is described in the present paper. For details of the experiments we refer to our other publications. Some basal internodes of vegetative tillers of P. pratense L. cv. Prato, treated with ethephon appeared to consist of two parts: a short, comparatively thick lower part, with a tiller sprouting just at the top of it (plate IC and ID, 3a) and a longer thinner upper part (plate IC and ID, 3b). The tiller occurred on the same side of the stem as the subtending leaf and in the axil of this leaf the bud was lacking. So it seems likely that it was the axillary bud, which after moving upwards had given rise to the tiller, and not an adventitious bud. In this case the “lower” part of the stem, between the tiller and the insertion of the lower leaf (plate IC and ID, 3b) should be considered as a part of the node, which normally does not elongate. Only the “upper” part, the part above the tiller, would be the internode. Support for this idea was sought by studying the course of the vascular bundles in a series of transverse and longitudinal stem sections. In the “upper” part the vascular bundles were arranged practically in a single ring, as is typical for Poa (Stiff & Powell 1974); but the “lower” part showed two adjoining rings with a few vascular bundles in common. Close to the region were the tiller had developed from the stem the latter bundles started to ramify (plate II). The course of the anastomoses showed a close resemblance to that in the nodes of untreated plants.