The juvenile period, i.e., the time between seed germination and when the plant achieves and maintains the ability to flower, is very prolonged in woody species (Hackett, 1985). The length of this period is determined by genetic and environmental factors (Kazakov and Kichina, 1988), and their interaction can generate different plant behaviors. In the olive tree (Olea europaea, L.), the juvenile period can last up to 15 years in natural conditions (Bellini, 1993), and has represented one of the major obstacles for breeding programs in this crop species. Moreover, the length of the juvenile period has agronomic importance due to its relationship with the length of the unproductive period of the new cultivars subsequently reproduced by vegetative cuttings, an important character for fruit trees. León et al. (2007) clearly demonstrated that olive seedlings with a short juvenile period will, once vegetatively propagated, also produce plants with a short unproductive period. Two main strategies have been used in fruit breeding programs to overcome these difficulties: 1) the development of early selection criteria to eliminate genotypes with a long juvenile period and 2) the shortening of this period by means of forced growth of seedlings.
Criteria for early selection of seedlings with a short juvenile period have been established for tree crop species such as apple (Alston and Bates, 1980; Visser et al., 1976; Zagaja and Faust, 1983), pear (Bagnara et al., 1994; Visser et al., 1976; Zimmerman, 1977) and, more recently, olive (De la Rosa et al., 2006; Pritsa et al., 2003; Rallo et al., 2008). In this latter species, early elimination of plants with a long juvenile period is based on plant height measured just before the time of transplanting to the field, excluding all seedlings shorter than a threshold height (De la Rosa et al., 2006; Rallo et al., 2008).
The second approach used to overcome the long juvenile period of fruit tree seedlings has been to force seedling growth, particularly to rapidly achieve height, in the greenhouse (Aldwinckle, 1975; Fischer, 1994) and/or field. In olive, the application of growth forcing and training has allowed a considerable reduction of the juvenile period, facilitating breeding program success (Lavee et al., 1996; Santos-Antunes et al., 2005). A number of growth-forcing strategies can be carried out. Santos-Antunes et al. (2005) forced growth in the greenhouse for 18 months and then planted the seedlings in the field with an average height of 160 cm. This procedure, however, entails management difficulties such as a marked imbalance between a very tall shoot and a pot-restricted root volume before planting in the field and labor-intensive staking and pruning following transplant. On the contrary, Lavee et al. (1996) focused on forcing growth in the field, where seedlings were planted when they were only 30 cm high. Under field conditions, an adequate growth-forcing strategy is not limited by root volume, plant space, or other management considerations.
In this study, we evaluate the influence of olive seedling transplant date from three different perspectives: early attainment of the capacity to flower, simplification of plant management, and rapid elimination of less desirable genotypes. We integrate the two main strategies that have been used to overcome the juvenile period, simultaneously evaluating the effect of a range of heights at the time of transplant to the field on the percentage of plants with a short juvenile period and testing the applicability of the early selection criterion for juvenile period (De la Rosa et al., 2006) to different seedling ages. In particular, we attempt to define more precisely and optimize the transplant times that can be used successfully, using plant height, shown to be an effective parameter for both strategies, as our principal measurement.
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