Dormancy is an adaptation of perennial plants to cease growth in response to extreme environmental conditions (Rohde and Bhalerao, 2007). Dormancy is induced by diverse factors in different plants (Vegis, 1964), whereas dormancy termination is usually influenced by chilling in winter (Fuchigami et al., 1982; Perry, 1971; Wareing, 1956). Temperature and duration of chilling are key components of satisfying chilling requirements which are critical to promote vegetative growth and budbreak for perennial plants in the subsequent year (Perry, 1971). Temperatures near 5 °C are required to satisfy the chilling requirement of perennial plants (Perry, 1971). However, the duration requirement of chilling varies considerably between and within species (Alburquerque et al., 2008; Brundell, 1975; Egea et al., 2003; Ruiz et al., 2007; Wall et al., 2008). The Richardson’s CU is often used to determine chilling requirement (Richardson et al., 1974). For example, the chilling requirements ranged from 266 to 996 CU among almond cultivars (Egea et al., 2003) and ranged from 800 to 1200 CU among apricot cultivars (Ruiz et al., 2007). Negative effects that are caused by insufficient or exceeded chill accumulation could result in reduction of budbreak or earlier flowering, which threaten the cultivation and commercialization of the perennial crop. Thus, finding the chilling requirements of plants is vital for the basic understanding of dormancy and practical application of dormancy release or bloom delay techniques (Naor et al., 2003).
Kiwifruit (Actinidia) is a functionally dioecious liana with variable ploidy levels (e.g., diploid, tetraploid, and hexaploid) (Huang et al., 2000; Li et al., 2010). Most current kiwifruit cultivars are selections of natural Actinidia chinensis and Actinidia deliciosa from China (Ferguson and Huang, 2007). Compared with other fruit trees, limited information is available about the effects of temperature on the chilling requirement in the Actinidia genus. It has been reported that Actinidia rufa displayed a low-chill trait (200 h) followed by A. chinensis (400–600 h), and A. deliciosa cultivars showed the highest chilling requirements (600–800 h) (Kulthinee et al., 2004). In addition, there is an obvious variation of chilling requirements among various cultivars of an Actinidia species. For example, ‘Hayward’ (A. deliciosa) had a chilling requirement of more than 950 CU for vegetative buds and 1150 CU for floral buds, whereas ‘Bruno’ (A. deliciosa) may have lower chilling requirements (Caldwell, 1989). However, information has not been reported about the chilling requirements of other kiwifruit cultivars, especially for the new released cultivars with diverse ploidy levels.
Because of China’s fluctuating climate and diverse environment (Luedeling, 2012), the appropriate chilling requirements of kiwifruit cultivars are essential for cultivar selection and cultivation. The objectives of this study were to 1) determine chilling requirements for vegetative budbreak and floral emergence of different kiwifruit cultivars and 2) investigate the relationship of chilling requirements with different ploidy levels.
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