Adaptation is a concept related to how plants can survive and reproduce in a specific environment (Hill et al., 1998), and it is reflected in the synchronization between the development stages and climate (Dietrichson, 1964). Chilling requirement is an aspect necessary to ensure that the plants are adapted to the appropriate area. Dormancy and freezing tolerance are the main mechanisms developed against very cold conditions, although they could be independent (Irving and Lamphear, 1967). Freezing tolerance cannot be developed adequately without growth cessation (Fuchigami et al., 1971), which marks the onset of dormancy. The interest in understanding the mechanism of dormancy set and release is mainly based on the necessity of manipulation of the dormant period to avoid spring frost damage (Faust et al., 1997). Dormancy has a significant economic impact on the maintenance and production of herbaceous and woody plants (Fennell, 1999).
The study of the phenological behavior of crops, as part of a well-characterized environment, is important both to obtain satisfactory production and to determine the most suitable agronomic techniques (Valentini et al., 2001). In this way, if a cultivar is established in an area where its chilling requirements are not satisfied adequately, the vegetative and reproductive growth of the cultivar will be affected negatively (Black, 1952; Coville, 1920; Ruck, 1975; Samish, 1954; Weldon, 1934). On the contrary, in the case of a cultivar with low chilling requirements growing in cold winter areas, the blooming happens too early because the chilling requirement is quickly satisfied (Scorza and Okie, 1990). Furthermore, studies concerning chilling and heat requirements are thus of special interest in these species, being very important for the choice of parents in breeding programs looking for late-flowering cultivars (Spiegel-Roy and Alston, 1979). When the chilling requirement is satisfied, blooming will start. Early blooming increases the likelihood of damage by late winter or early spring frosts. The time of full bloom depends on two factors: the chilling requirements and growing degree hours Celsius (GDH °C) required after endodormancy for reaching full bloom (Raseira, 1986).
The risks related to the lack of knowledge for the heat requirements of walnut cultivars is less than the chilling requirement, but the knowledge will provide us with more possibilities for the management of this crop so that cultivars with low chilling requirements but high heat requirement could be cultivated in relatively cold areas (Citadin et al., 2001). Methods for determining the heat requirements of blooming have been developed (Richardson et al., 1974). These methods essentially consist of establishing the heat accumulation, above a threshold, to which a tree is exposed from breaking of dormancy until flowering date.
The chilling requirements for the termination of endodormancy (rest) were studied by many researchers (Chandler et al., 1937; Coville, 1920; Molisch, 1908). There are several methods for estimation of chilling requirements of plants such as using detached twigs (Citadin, 1999; Citadin et al., 1998; Herter et al., 2000), individual buds (Bianchi et al., 2000; Herter et al., 1992), and plants in containers (Camelatto et al., 2000; Citadin et al., 2001).
Each tree species has a specific chilling requirement that is related to the accumulated hours below a chilling temperature threshold or to cumulative chill unit, which are hours that are weighted for temperature effectiveness for breaking dormancy (Erez et al., 1979; Weinberger, 1950). Although it has been mentioned that many walnut cultivars require ≈400 to 1500 h temperature below 7 °C to fulfill chilling requirements (Chandler et al., 1937), the available data for the chilling and heat requirements of walnut cultivars, especially for domestic walnut genotypes, are scarce. Therefore, the aim of this investigation, in 2 successive years, was focused on calculation of chilling and heat requirements to break bud dormancy of some cultivars and domestic walnut genotypes using 1-year-old twigs picked in the fall. Hence, the information obtained will provide better understanding of chill and heat requirements of walnut, which will be useful for best cultivation of the cultivars in the proper region.
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