Agriculture Reference
In-Depth Information
The breaking of isolated apple buds in tissue culture is inhibited by abscisic
acid (ABA) (Borkowska,
), even if the
chilling requirement of the buds is fully satisfied. The effect of ABA is, how-
ever, only at a late stage of the bursting process, i.e. it inhibits extension of
the bud axis. Edwards (
; Borkowska and Powell,
/
) found that defoliation of apples in the trop-
ics, triggering budburst, was accompanied by a decline in bud ABA content
which he attributed to the removal of the source of their ABA, i.e. the mature
leaves.
Early investigators of correlative inhibition postulated that the apical bud
monopolized nutrients to the detriment of lateral buds. This is no longer
considered to be the prime mechanism of apical dominance although nutrient
and water status may influence the process. Phillips (
) noted that inorganic
nutrient deficiency, particularly of nitrogen, can increase the strength of apical
dominance and depress cytokinin levels.
Tromp (
a, b) found that apical dominance in young apple trees is greater
at low soil temperatures and low atmospheric humidity. He attributed this
to more adverse water status and possibly reduced supplies of root-produced
cytokinins.
Correlative inhibition of buds is generally enhanced at low light intensity,
perhaps as a result of lower photoassimilate and higher auxin levels (Phillips,
).
Gravity regulates bud growth and lateral organ orientation. Placement of
a normally negatively geotropic (i.e. upward-growing) shoot at or near to the
horizontal usually reduces correlative inhibition of buds by the main apex.
This may relate to the higher auxin content of terminal buds of vigorous up-
right shoots than of terminal buds of horizontal or weeping shoots in apple
(Kato and Ito,
). In general, the physically highest and upwardly directed
bud on a shoot achieves dominance. When all buds are at equal height as
on a horizontal branch, the upwardly directed bud nearest to the root system
is the most likely to become dominant. Under tropical and subtropical con-
ditions all the buds on the upper side of a horizontal branch often break to
give short fruiting spurs. In general, under the influence of gravity, the IAA
levels are higher in the lower than the upper halves of horizontal plant or-
gans (Reinhold,
b) found that NAA
applied to the cut end of a horizontal apple branch moved mainly to the
lower half of this, as shown by development of characteristic auxin-induced
xylem.
). Blanco-Brana and Jackson (
