Abstract
The purpose of this work was to study the response of two apple cultivars bred for low chilling environments to artificial chilling accumulation. Two trials were carried out; in experiment one, excised shoots were randomly taken from “Caricia” and “Princesa”, and in experiment two, intact and excised shoots of “Caricia”, “Princesa” and “Gala” (control) were collected. After collection, both shoot types were exposed to artificial chilling accumulation (4.0 ± 0.5°C) from 0 to 1200 chill units (CU). Bud break of mixed buds of “Caricia” and “Princesa” was higher than 50% between 0 CU to 1200 CU, irrespective of shoot type. Bud break of “Gala” mixed buds exceeded 50% only in intact shoots after accumulating 900 CU. The mean time to bud break of “Caricia” and “Princesa” diminished with increasing chilling accumulation and stabilized after ~600 CU, depending on the type of shoot and the year of experimentation. The low-chill apple cultivars tested in this work showed shallow dormancy, but they required moderate cold accumulation (800 – 1150 CU) to fully satisfy their chilling requirements. Thus, although their shallow dormancy makes them suitable for cultivation in chill-deficient environments, they cannot be considered low-chill cultivars.
Highlights
Apple production in chill-deficient environments became possible with the release of low-chill apple cultivars (Hauagge & Cummins, 2001)
In experiment 1, the mean time to bud break (MTB) of lateral mixed buds decreased significantly with increasing chilling accumulation, but the response differed between the varieties, with a highly significant interaction between the factors cultivar and chilling (p = 0.001, r2 = 0.59)
The variation in MTB in response to chilling accumulation was fit to a cubic function on both apple cultivars, a stabilization point at approximately 800 chill units (CU) was found according to our criterion (Figure 1A)
Summary
Apple production in chill-deficient environments became possible with the release of low-chill apple cultivars (Hauagge & Cummins, 2001). Low-chill apple production has since been extended to the tropical and subtropical regions of Latin America, Africa and Asia (Ashebir et al, 2010; Castro, Cerino, Gariglio, & Radice, 2016; Mohamed, 2008; Njuguna, Wamocho, & Morelock, 2004; Pommer, & Barbosa, 2009). Agronomy the Americas, novel cultivars, such as “Caricia” and “Princesa”, are becoming important in mild-winter areas either for cultivation or as genetic material for apple breeding programs (Pommer, & Barbosa, 2009). One of the major challenges of temperate-zone fruit production in warm-winter areas is to overcome the dormancy period (Erez, 2001). Dormancy has been defined as the inability to initiate growth from meristems or other organs and cells, with the capacity to resume growth under
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