Agriculture Reference
In-Depth Information
(Powell,
) and to dominance effects of young expanding leaves (Abbott,
).
Initially,thedevelopmentofdormantbudsisreversibleifgrowingconditions
improve and a new flush of growth can occur following heavy summer rain,
the application of nitrogenous fertilizer in summer or early summer pruning.
Abbott (
) concluded that effects of stress leading to shoot growth termina-
tion and the temporary reversal of these are mediated by effects on root growth
and the supply of growth substances from the roots. Alternatively, auxin or
inhibitors may be involved.
Development of deep dormancy,
endodormancy or rest
As summer, autumn and early winter progress in the temperate zone, apple
and pear terminal buds enter a state of endodormancy (Lang,
). In this
state they cannot be stimulated into rapid budbreak by transfer into condi-
tions of temperature and water supply suitable for normal growth, even in
the absence of external sources of correlative inhibition. The 'depth' of such
endodormancy, otherwise referred to as true dormancy, deep dormancy or
rest, is usually measured in an arbitrary way as the number of days at a tem-
perature in the range
◦
C, that are needed to induce a given stage of bud
development (Spiegel-Roy and Alston,
-
).
At Geneva, New York State, some cultivars show appreciable endo-
dormancy even on
; Saure,
July and most cultivars show significant levels of such
dormancy before any winter chilling or winter frost occurs (Hauagge and
Cummins,
b). Cultivars can be divided into three categories based on the
number of days of 'forcing' required to induce
% of shoots to show termi-
nal budbreak. Cultivars in the first category, which include 'Anna', 'Dorsett
Golden' and 'Ein Shemer', show only shallow dormancy throughout. They
give
◦
C at any time in
the season. These cultivars show excellent adaptation to subtropical climates
with mild winters (Miller and Baker,
% budbreak with fewer than
days 'forcing' at
; Bepete and Jackson,
). Jackson
(
) reported that in tropical Zimbabwe 'Anna' can produce two crops a
year without any defoliation and without any prior exposure of the flowers of
the second crop to chilling. This is compatible with the conclusion of Hauagge
and Cummins that these cultivars never exhibit deep dormancy. The majority
of standard, temperate-zone cultivars show a very different pattern, character-
ized by abrupt changes in the intensity of dormancy and very deep dormancy
(Figure
September, before
the onset of cold weather, others not until after the first frosts and the begin-
ning of accumulation of chilling units (see p.
.
). Some cultivars reached deep dormancy by
). Maximum dormancy for
most cultivars occurs at the time of complete leaf senescence, as based on leaf
