lier)—they are prevented from growing by the presence of the apex rather than by conditions within the
buds themselves. As the summer progresses, the ability of the buds to grow following apex removal de-
clines; paradormancy is gradually becoming endodormancy as control shifts from the apex to the buds
themselves. By the end of the season, the buds no longer respond to apex removal; endodormancy is now
fully established.
Many woody perennials (e.g., birch) exhibit a marked response to photoperiod, growing rapidly un-
der long photoperiods, slowly or not at all under short photoperiods. This response is truly photoperiodic
rather than being a function of total time of exposure to light per se and is an example of ecodormancy.
When plants are grown under short days but the long night is interrupted by a brief period of light, they
continue their growth. Under natural conditions, the effects of long days are often masked by other envi-
ronmental limitations, such as water supply or competition among growing points. Thus mature trees of
birch stop growth in midsummer, even though daylength is near its maximum.
Chilling temperatures appear to be required for buds to become fully endodormant. In some areas of
the tropics and subtropics where temperatures never fall below 20°C, the buds of peaches, grapes, and ap-
ples can be forced to grow by defoliation soon after harvest. This permits production of two or more crops
per year. The longer the interval between harvest and defoliation, the poorer the response. Trees that are
not defoliated may eventually become endodormant; in the absence of chilling, they cease growth entirely
and eventually die.
Endodormancy is normally broken by exposure to chilling temperatures. Optimum temperatures
vary with species but generally range from 0 to 10°C; temperatures below 0°C have little or no effect.
Considerable research has been done to determine the chilling requirements of fruit tree species and cul-
tivars, and several models have been developed to predict when these requirements have been satisfied.
For example, according to the Utah model [49], the number of chill units required for ‘Elberta’ peach and
‘Delicious’ apple are 800 and 1234, respectively [50]. A chill unitis defined as 1 hr of exposure to a tem-
perature of 6°C; higher and lower temperatures between 0 and 13°C are less efficient, and temperatures
above 13°C are inhibitory; thus adjustments must be made in calculation (Figure 6). This model, devel-
oped in the north temperate zone, may not apply in regions where diurnal temperature fluctuations are
greater. Israeli scientists have therefore developed a “dynamic” model in which temperatures alternating
between about 6 and 13–14°C are considered to have a greater effect than continuous cold in breaking
dormancy [52]. Temperatures above 15°C are inhibitory unless the exposure time is less than a critical
length. This model was more effective than the Utah model in predicting end of rest when used in IsraelDORMANCY: MANIFESTATIONS AND CAUSES 169Figure 5 Degree growth state model representing stages in the annual cycle of growth in woody plants. Five
sequential growth stages [spring budbreak (SBB), maturity induction point (MI), vegetative maturity (VM) (
onset of endodormancy), maximum endodormancy (MR), and end of endodormancy (ER)] occur at 0, 90, 180,
270, and 315°C, respectively. (From Ref. 48.)