8.1.3 Bud dormancy
Much of our knowledge of bud dormancy comes from temperate deciduous trees, especially fruit crops such as apples and stonefruit. Trees detect environmental signals, mainly shortening daylength and cold, which herald winter and trigger reductions in growth rate, onset of endodormancy, development of bud scales and leaf fall. As buds enter endo-dormancy, warm temperatures (>15°C) no longer promote growth. Several weeks or months of chilling (0–12°C) are required to overcome endodormancy. The plant then enters ecodormancy, when it will respond to warm temperatures with bud break. Note that break of endodormancy can therefore often occur weeks prior to growth resumption. In some tropical species such as coffee, water stress is an alter-native cue for breaking flower bud endodormancy (Drinnan and Menzel 1994). Buds then exist in an ecodormant state ready to respond by rapid floral growth as soon as the ﬁrst rains fall at the end of the dry season (Figure 8.2).
Several models have been proposed to describe dormancy and to attempt to predict responses to different growing con-ditions. One problem is a lack of measurable indicators of endodormancy other than an inability to grow. Researchers typically quantify ‘depth’ of dormancy by the duration of chilling required to break dormancy, and then the ability of warm temperatures to ‘force’ bud growth on cut shoots, that is, after removing possible causes of ecodormancy and paradormancy. Entry into and exit from bud dormancy are often gradual transitions rather than abrupt events. Some researchers have represented these phases as sine wave oscillations, with measurable reference points (e.g. peak growth rate in summer and maximum dormancy in midwinter) which enable comparison of data from different sites (Fuchigami and Nee 1987).
Temperate crops in the tropics
Temperate fruit crops are increasingly being grown at lower latitudes (15–30°) than where they originate (30–50°). If endodormancy is still being overcome by chilling, then how little chilling is enough? A good model can allow estimation of whether a new location is suitable for production of particular fruit varieties prior to expensive orchard planting. For example, peach and nectarine varieties have been bred with low and high chilling requirements, suited to subtropical and temperate climates respectively. Early models resulted in rankings based on number of chill hours (usually below 7.2°C). Chilling required can vary from less than 50 h below 7.2°C for some subtropical ‘low-chill’ peach cultivars, up to 3000 h for some cultivars of pear (Table 8.4). A modiﬁed version, called the Utah model, equates a chill unit to 1 h at 6°C; higher and lower temperatures between 0–15.9°C have proportional positive effects, but temperatures above 16°C are inhibitory (Richardson et al. 1974). This temperate model is less accurate in warmer areas where the Erez et al. (1988) model, as modiﬁed by Batten and Firth (1987), often provides a more reliable estimate of date of budburst (Table 8.5). According to this model, effectiveness of chilling is enhanced by day temperatures of 15°C or less but negated by temperatures above 18°C. None of these models quantify the growth-permitting periods of warm temperature required for subsequent bud break, so an additional measure quantiﬁes thermal units: the Growing Degree Hour where 1h is allocated for each hour and degree above 4.5°C (Figure 8.3).
What are the consequences of insufﬁcient chilling, and are there alternative treatments? Symptoms of inadequate cold periods include delayed and weak leaf growth, delayed and protracted flowering, poor fruit development and irregular ripening. Potassium nitrate (KNO3), thiourea and especially hydrogen cyanamide are simple chemicals that are effective substitutes for stimulating uniform budburst. The mechanisms by which these compounds work are not known, but growth regulators such as gibberellins, cytokinins and cytokinin analogues, in particular thiadiazuron, can also cause similar responses.
Apples are grown in the tropical and subtropical areas of Indonesia, peaches are grown in Venezuela and table grapes are grown in Thailand, Venezuela and southern India where no chilling occurs (Subhadrabandhu and Chapman 1990). Growth of buds is stimulated by chemical (sodium chlorate, copper sulphate or urea) or manual defoliation or pruning immediately after harvest thus breaking endodormancy before it enters its ‘deep’ midwinter phase. Cyanamide treatment has enabled out of season production of table grapes in tropical Queensland. Irrigation then promotes uniform budburst and cropping under otherwise dry conditions. At least two harvests are possible each year and cycles can be staggered, giving almost continuous fruit supply.