Fruit set assessment is currently the most commonly used method to test cross-compatibility among olive (Olea europaea L.) cultivars, thus identifying the best pollenizers (Androulakis and Loupassaki, 1990; Camposeo et al., 2012; Cuevas et al., 2001; Farinelli et al., 2002a, 2006, 2008a; Fernandez-Escobar and Gomez-Valledor, 1985; Guerin and Sedgley, 2007; Lavee, 1998; Lavee and Datt, 1978; Moutier, 2002; Rallo et al., 1990; Tombesi et al., 1982). To our knowledge, only a few researchers have investigated cultivar compatibility by paternity tests with simple sequence repeat markers (Diaz et al., 2007; Mookerjee et al., 2005; Rodriguez-Castillo et al., 2009; Rosa de la et al., 2004).
Moreover, there have been very few studies on the effects of different cultivar/pollenizer combinations on seed and drupe characteristics (Cuevas and Oller, 2002; Farinelli et al., 2002b, 2008b; Koubouris et al., 2010). Olive fruit is a drupe, consisting of the carpel, botanically similar to almond, apricot, cherry, nectarine, peach, and plum. The ovary wall has both fleshy and hard portions (Martin et al., 2005). A drupe usually contains a single seed enclosed by a hardened endocarp, which often adheres closely to the seed. Although olive flowers have four normal ovules, usually only one of them will be fertilized, giving origin to the seed. The fruit rarely contains two normal seeds (Cuevas et al., 1995), although there are some cultivars that may set fruit containing two normal seeds at a higher frequency (Cuevas and Oller, 2002), whereas others may not contain any seeds because of embryo death (Morettini, 1950; Rapoport and Rallo, 1990). It is important to note that the presence of a seed or seeds is very significant because the seed influences fruit development during the current year and the extent of flowering in the next annual olive cycle (Koubouris et al., 2010; Lavee, 2006; Rapoport, 1994).
In olive, like in other fruit, the number of seeds per fruit influences fruit and seed weight (Cuevas and Oller, 2002; Farinelli et al., 2002b; Stafford, 1970). Fruit containing two seeds is usually larger because of greater mesocarp and endocarp growth.
This aspect is very important in table olive cultivars, which must produce good-sized fruit with a high pulp/pit ratio. This is also important in olive oil cultivars because fruit with no normal seeds is smaller and contains less oil (Morettini, 1950). Furthermore, the absence of a normal seed in the fruit has an economic impact on propagation in olive nurseries, when reproduction methods are used to obtain seedlings destined for use as rootstocks for grafting. In fact, seed characteristics can affect germination and seedling quality. Also, competition among fruit is thought to be proportional to sink size, which in turn is related to fruit and seed mass, but not to fruit number (Farinelli et al., 2002b; Rosati et al., 2010).
In many fruit growth models such as the “Peach” model, sink strength is calculated taking into account the sink potential, which is proportional to fruit mass (Grossman and DeJong, 1994, 1995; Marcelis and Heuvelink, 1999). In olive, the percentage of fruit set, the percentage of undeveloped seed, and the number of drupes containing two seeds seem to be affected by the affinity between the cultivar and the pollenizer (Cuevas and Oller, 2002; Martin et al., 2005).
More in-depth studies on the relationship between cultivars and pollenizers in determining fruit set, embryo death, seed number, and development and fruit growth are needed as well as evaluation of the maternal and paternal influences.
In the present study, the effect of all possible pollination combinations among the cultivars Ascolana Tenera, Carolea, Leccino, and Picholine and other pollenizers on the number and quality of seeds and their development and drupe and seed characteristics was evaluated using the χ2 test of independence.
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