Agriculture Reference
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The obvious effects of the terminal bud and of branch bending on lateral bud
endodormancy indicate that auxin can influence chilling requirements and in-
crease the depth of endodormancy. Auxin (indolebutyric acid, IBA) applied to
apple roots increased budbreak (Young,
), presumably as a consequence
of its effect in increasing new root growth which is likely to have led to in-
creased cytokinin production. Tying down to give horizontal branches as part
of the technique for dormancy control in the tropics presumably involves
auxin-mediated effects.
Abscisic acid (ABA) was originally thought to be the most important inhibitor
preventing growth and to be a causal factor of dormancy, whether drought-
induced or developed in anticipation of winter (Faust et al. ,
). ABA is
effective in delaying budbreak in tissue-cultured shoots of apple (Dutcher and
Powell,
) and the ABA content of apple buds declines through the winter
(Borkowska and Powell,
/
), reaching a minimum level at the time of
budbreak(SeeleyandPowell,
).Thereisnoclearevidenceofalinkbetween
ABA and chilling effects (Powell,
) concluded
that ABA may be responsible for all or part of the effect of bud scales in
inhibiting budbreak, and Taylor et al. (
). However, Swartz et al. (
) found that
defoliation under tropical conditions was followed by a decrease in bud ABA
content followed by budburst. Hydrogen cyanamide treatment, which can
overcome winter dormancy in the tropics, induces a reduction in ABA levels in
both apical and lateral buds of apple, followed by budburst (Subhadrabandhu,
) and Edwards (
). It is possible that ABA is involved in dormancy through its effects on
dehydrins and changes in membrane permeability (Faust et al. ,
).
Broome and Zimmerman (
) showed that externally applied cytokinins
were at least as effective as chilling in inducing budbreak of otherwise dor-
mant lateral buds of Malus hupahensis . Their plants were seedlings grown in
continuous long days in a greenhouse so the dormancy, though responsive
to chilling, may have been atypical. When Thidiazuron (TDZ), a potent and
stable compound with cytokinin-like activity, was applied prior to winter chill-
ing of shoots from 'Delicious' and 'Northern Spy' it markedly reduced their
chilling requirement for budbreak (Steffens and Stutte,
). It had a similar
but smaller effect on 'Anna'.
Theconcentrationofcytokininsinthexylemsapoforchardtreesof'Granny
Smith' increases steadily during the sixweeks prior to budbreak, followed by
a rapid increase immediately before and at budbreak, followed by a decline
(Cutting et al. ,
). Treatment with the rest-breaking agents dinitro- o -cresol
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