Agriculture Reference
In-Depth Information
under tropical conditions in Indonesia. Removal of mature leaves under tropi-
cal conditions is followed by budburst of terminal buds on spurs ( Janick,
;
Edwards and Notodimedjo,
).
Mechanisms of correlative inhibition
There is no single theory that provides a fully satisfactory explanation of the
mechanism of correlative inhibition (Hillman,
). There is, however, con-
siderable evidence that apically produced auxin indirectly suppresses axillary
bud outgrowth that is promoted by cytokinin originating from roots or shoots
(Cline,
). Other hormones may also be involved and there
may be a critical role for nutrients and for water as a possible inducing signal
for bud outgrowth.
; Tamas,
"
Thimann and Skoog (
) originally demonstrated the presence
of auxin in apical buds of
Vicia faba.
They showed that the amount of
inhibition of budbreak of laterals was related to the auxin content of the
apical bud and that indoleacetic acid (IAA) applied to a decapitated stump
can to some extent substitute for the apex. Wang
et al.
(
,
) quantified the
concentration of IAA needed to substitute for apple shoot apices. They found
that exogenous IAA inhibited the increase of free water in lateral buds that is
an initial step in their release from dormancy. Abbas (
) showed that the
shoot tips of apple cultivars that are freely branching have lower concentra-
tions of auxin-like substances than those of cultivars showing little branching.
The application of an auxin transport inhibitor below the apical zones of intact
apple shoots induces budbreak of the hitherto dormant lateral buds below the
treated zone (Duckworth
et al.
,
). The emergence of shoots from dormant
trace buds in the tissues of secondarily thickened branches following pruning
can be prevented by painting IAA or the synthetic auxins IBA or NAA
on the pruning cuts (Blanco-Brana and Jackson,
a), as shown in
Figure
.
The most rapid synthesis of auxin occurs in expanding leaves (Sachs,
.
),
but it also occurs even in mature leaves and other parts of shoots. Dominant
organs, which are usually the first to develop, have a high IAA export rate
while reducing the auxin export by inhibited organs.
Studies on auxin movement tend to emphasize polar transport. In shoots
this is basipetal, with auxin moving preferentially from morphologically apical
to more basal regions. In roots it then continues to move in the same physical
direction as in the shoots, but this is now called acropetal movement because
it is in the direction of the root tip. In young roots, however, there may also be
movement away from root tips. This polar transport is specific for IAA and
