CHANGES IN EPILOBIUM PHYLLOTAXY INDUCED BY N‐1‐NAPHTHYLPHTHALAMIC ACID AND α‐4‐CHLOROPHENOXYISOBUTYRIC ACID

Abstract

Preliminary studies establishing relationships between leaf plastochron index and Epilobium hirsutum L. shoot growth provide a method for rigorous selection of plants utilized in experiments designed to test the working hypothesis that endogenous auxin gradient interactions are factors of phyllotactic control in this species. Application of Nsl-naphthylphthalamic acid (NPA), an auxin transport inhibitor, to one of the youngest bijugate primordia on the shoot meristem results in increased growth of the treated primordium. Fasciation between the treated primordium and one of the next primordia to be initiated alters relative vertical spacing of primordia. Angular shifts between subsequent primordia result in spiral transformation of Epilabium bijugate phyllotaxy. Application of a-4-chlorophenoxyisobutyric acid (CPIB), an auxin antagonist, to one of the youngest bijugate primordia on the shoot meristem results in decreased growth of the treated primordium that alters both radial and vertical spacing of primordia. This is followed by angular shifts between subsequent primordia resulting in spiral transformation of the bijugate phyllotaxy. Changes in the growth parameters of NPA-and CPIB-treated shoots are similar. Relative plastochron rates of radial and vertical shoot growth of induced spiral shoots are about half those of lanolin paste control shoots, as are the plastochrons and relative plastochron rates of leaf elongation. Treated shoot meristems have eccentricities of 0.5 as compared to bijugate control meristem eccentricities of 0.7. No significant difference is apparent between basal transverse areas of treated and control shoot meristems. The relative chronological rates of growth of treated shoots are not significantly different from those rates of control shoots. Spiral transformation results from changes in relative positions ofleafprimordia insertion on the shoot meristem, not from changes in growth of treated shoots. These changes are accompanied by an increased rate of leaf initiation on a more circular shoot meristem. Existing theoretical models of phyllotaxy are discussed in relation to these chemically induced changes of Epilobium leaf arrangement.

The shoot apical meristem: the dynamics of a stable structure

“…When applied to young primordia, NPA, an inhibitor of auxin transport, induces their lateral extension (Meicenheimer 1981). A similar effect was obtained on primordia of Phaseolus using the synthetic auxin 2,4-D (Pereira & Dale 1982).…”

Organ positions and pattern formation in the shoot apex

“…These parameters can be obtained from transverse sections through the shoot apex (Maksymowych and Erickson, 1977;Richards, 1951) or from SEMs (Meicenheimer, 1981). When using transverse sections, it is important that the section not be oblique.…”

“…Examples of such factors include the internodal distance between primordia at the shoot apex and the size of a primordium at initiation relative to the size of the meristem. In fact, some studies have shown that changes in these parameters are associated with changes in phyllotaxy (Greyson and Walden, Greyson et aL, 1978;Maksymowych and Erickson, 1977;Meicenheimer, 1979Meicenheimer, , 1981. Although changes in these parameters are associated with changes in organ positioning, whether or not they play a key role in determining phyllotaxy or are simply the result of phyllotaxy has yet to be determined.…”

Foliar modifications induced by inhibition of polar transport of auxin

“…And it was demonstrated that auxin polar transport was essential for the establishment of bilateral symmetry during the development of zygotic embryos [3]. In particular, some reports suggested that disruption of auxin polar transport resulted in aberrant organ formation at the shoot apex [4][5][6][7][8][9][10][11]. When the tobacco leaf and pedicle explants were cultured on a medium containing auxin polar transport inhibitors, the formation of trumpet-shaped leaf-like organs was observed [4], [5].…”