|dc.description.abstract||Adventitious rooting in stem cuttings of Eucalyptus grandis Hill ex Maid. was thought to
be influenced by a putative inhibitor. In previous studies it has been usual to infer the
presence of putative rooting inhibitors and promoters from the mung bean bioassay, but
the possibility was raised that treatment responses in this assay could be mediated more
by the concentration of the treatment solution than by the chemical identity of the solute.
This appeared to be so: several solutes, including hydrochloric acid and common salt,
were found to promote the rooting of mung bean cuttings when present in the treatment
solution at an apparently injurous concentration. The concept of promoters and
inhibitors of adventitious rooting, as constituted at present, was considered to be an
unfavourable approach for further studies.
Stem cuttings must contain a morphogen, broadly defined, which operates the 'switch'
from stem to adventitious root. The leaves and buds of E.grandis stem cuttings did not
appear to be sole sources of a morphogen (as is often assumed), but nevertheless the
activity of the leaves and buds was good for rooting. This activity was reflected in the
pattern of root emergence. A slight preponderance emerged from the leaf trace sectors
of the stem, suggesting that the leaves and buds cause a morphogen (of unknown origin)
to circulate in the cutting.
The existence of a vascular morphogen was confirmed and it proved to be very mobile
in the stem, suggesting that it is well distributed circumferentially at the base of the
cutting rather than confined to the leaf trace sectors. It appeared to be super-abundant
at the base of easy-to-root cuttings, but it was not possible to tell to what extent the
morphogen was rendered accessible to the sites where roots initiate.
In general, the rate of efflux from the transporting tissues, the rate of attenuation of the
morphogen after efflux, and the number of potential sites for root initiation must interact
on a small scale to determine rooting ability. The relative prominence of these groups of
factors would be expected to vary with circumstances, for example at different locations
within a single stem cutting, so the traditional concept of a limiting morphogen
('rhizocaline') is unhelpful in its simplest form.
Nevertheless, the rhizocaline concept provides a starting point towards a more comprehensive
view of adventitious rooting, which is required in order to predict and improve
rooting ability. This view remains a remote objective because many of the factors which
could be important have recieved very little attention and will be difficult to elucidate.||en