Agriculture Reference
In-Depth Information
synthetic auxins. It appears to be through parenchymatous cells, particularly
thoseassociatedwithordifferentiatingintovasculartissues,butnotthroughthe
vascularelementsthemselves(Rubery,
).Itrequiresenergyandisinhibited
by anaerobiosis and metabolic poisons such as cyanide and dinitrophenol. It
is also inhibited by specific auxin transport inhibitors such as
,
,
-tri-iodo-
benzoic acid (TIBA).
Both endogenous and applied auxins can also move in the plants' vascular
system. Auxin applied to mature leaves is translocated in the phloem like a
photoassimilate and may also move in the xylem transpiration stream. This
transport in the vascular system is by mass flow down gradients of osmotic po-
tential or water potential. Inactive auxin conjugates may move in the vascular
system and be activated by enzymatic hydrolysis before being distributed by
polar transport.
In apple, Soumelidou
et al.
(
) found predominantly basipetal but
also some acropetal movement in shoots. Blanco-Brana and Jackson (
b)
found that labelled NAA applied to the cut surface of a short shoot (spur)
growing vertically from a horizontal apple branch moved into the latter
and along it, mainly towards the trunk but with some acropetal movement
as well.
In many species, including apple, cytokinin treatment of buds releases them
from correlative inhibition (Sachs and Thimann,
), and a
relationship between endogenous cytokinin and axillary bud growth has been
demonstrated by comparing two tomato lines with different degrees of apical
dominance. Williams and Stahly (
; Tamas,
), Williams and Billingsley (
) and
many other authors have shown that the cytokinin
-benzyladenine (BA) can
be used to induce breaking of lateral buds of apple otherwise kept dormant by
apical dominance. Thidiazuron (TDZ), which has cytokinin-like properties, is
even more effective (Wang
et al.
,
).
Although cytokinins are thought to be mainly produced in the roots, there
is no need for contemporaneous root production of cytokinins to stimulate
budburst when a shoot apexis removed. This budburst happens even when
isolated shoots without roots are decapitated.
Gibberellins do not appear to be involved directly in apical bud dominance
(Tamas,
), but the outgrowth of axillary buds can be enhanced by the
application of GA
. The terminal budbreak of apples in the tropics which is
induced by leaf removal is preceded by a large increase in both concentration
and total amount of gibberellins in the buds (Taylor
et al.
,
; Edwards,
), as shown in Figure
.
.
