Biology Reference
In-Depth Information
addressed the question of whether phase change is regulated by factors from
the root system, by reciprocally grafting
E. tenuiramis
and
E. risdonii
. These
cross-compatible sister species differ primarily in the timing of vegetative
phase change;
E. tenuiramis
undergoes vegetative phase change at the 36th
node, on average, whereas
E. risdonii
remains permanently juvenile. Grafting
adult shoots from
E. tenuiramis
onto an
E. risdonii
root stock or juvenile
shoots of
E. risdonii
onto an
E. tenuiramis
root stock had no effect on the
timing of vegetative phase change in the scions, suggesting that the timing
mechanism is localized to the shoot.
In
E. tenuiramis
(
Wiltshire &Reid, 1992
), as in other plants (
Schaffalitzky
de Muckadell, 1954; Telfer et al., 1997
), the phase identity of a branch
matches the phase identity of the primary node from which it arises. Bra-
nches at juvenile nodes produce approximately the same number of juvenile
leaves as the primary shoot subsequently produces, whereas branches at adult
nodes immediately produce adult leaves. Because the lateral buds that pro-
duce branches arise
de novo
from stem/leaf tissue, this phenomenon suggests
that the developmental identity of a lateral bud is determined by the identity
of the cells from which it originates; that is, the identity of leaf or stem cells
determines the fate of a lateral bud. An alternative possibility is that the fac-
tors responsible for vegetative phase change act globally throughout the
shoot, simultaneously transforming both the primary shoot apex and lateral
buds and immature branches. For this to be true, branches at different posi-
tions on the shoot would have to be at different growth stages at the time of
vegetative phase change in order to account for the gradient in the number
of juvenile leaves produced by these branches.
The most striking result to emerge from this study (
Wiltshire & Reid,
1992
) concerns the behavior of epicormic buds. Epicormic buds are develop-
mentally arrested accessory meristems that form at the base of primary lateral
buds, and only emerge after the primary shoot is damaged or decapitated. In
contrast to primary lateral buds—which produce branches with slightly more
juvenile leaves than the primary shoot—epicormic buds from the base of
mature trees produce fewer juvenile nodes than the primary shoot. Wiltshire
andReid noted that these buds were present in an arrested statewhile the shoot
was undergoing phase change and concluded “although apical meristems have
an innate timing
...
other meristematic tissue (such as accessory buds) can be
influenced by the status of the whole plant.” This is the first indication that
vegetative phase change is associated with changes in the character of organs
and tissues throughout the entire shoot, not just within the shoot apex. It raises
the question of whether the signals that regulate this process act specifically on
