Agriculture Reference
In-Depth Information
(DNOC) oil and hydrogen cyanamide resulted in earlier and more rapid in-
creases in sap cytokinins and concomitant advancement and intensification of
budbreak. Tromp and Ovaa (
) similarly showed a sharp increase in the
concentration of cytokinins prior to budbreak of 'Cox' and 'Discovery.' Young
(
) found that increases in xylem cytokinins followed forcing, i.e. exposure
to high growing temperatures, irrespective of winter chilling. Cytokinin levels
then decreased significantly as budbreak occurred in fully chilled trees but they
did not in unchilled trees, which showed very little budbreak. When
-BA was
applied after artificial root and shoot chilling it increased budbreak by a fairly
constant amount whether the roots, or the shoots, or both, had been chilled
but did not induce budbreak of unchilled trees (Young and Werner,
).
The evidence therefore points to cytokinins having a major controlling role in
budbreak, adding to or amplifying the effects of chilling but not being able to
totally replace this. It should be noted, however, that the normal requirement
by apple seeds for a long period at chilling temperatures before they will ger-
minate can be totally replaced by soaking embryos in
-benzylaminopurine
(Zhang and Lespinasse,
). This may be significant in view of the many
parallels between seed and bud dormancy.
El-Banna et al. (
ppm GA applied during winter dor-
mancy in an environment with sub-optimal winter chilling in Egypt led to
advancement of vegetative budbreak and of flowering of 'LeConte' pears by
about two weeks. It increased vegetative budbreak from
) found that
%to
%, flower
budbreak from
fruits per tree. The
effects were very similar to, although rather more pronounced than, those of
a standard 'dormancy-breaking' treatment with DNOC.
Taylor et al. (
%to
% and yield from
to
) found that defoliation of apple trees
inIndonesiawasfollowedbyathree-foldincreaseingibberellin-likesubstances
and, subsequently, by budburst. Exogenous application of GA increased bud-
burst of both defoliated and undefoliated trees.
Although GA sometimes stimulates germination of dormant seed embryos
the effects are less consistent than those of
) and Edwards (
-benzylaminopurine (Zhang and
Lespinasse,
). Chilling appears to enhance the gibberellin content of seeds
and may induce a shift in free and bound forms of GA (Powell,
).
!
During dormancy water is closely associated with macromolecules and is in a
bound state (Faust et al. ,
). It is gradually freed during the winter dormant
period and rapidly converted to free water when resumption of growth is
triggered by TDZ or by forcing conditions. TDZ also induces a change from
bound water to free water in buds subjected to correlative inhibition.
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