**2. Ovary vs. fruit characteristics**

Fruit size is important commercially and the study of factors affecting it are of great scientific interest. Fruit size depends mostly on the size and number of fruit cells, though intercellular space may be also important (Bertin *et al.*, 2002; Corelli-Grappardelli, 2004). In olive, fruit size differs greatly among cultivars (Barranco, 1999). Both the endocarp and mesocarp contribute to final fruit size differences among cultivars (Hammami *et al.*, 2011).

Fuit size differences across cultivars are mostly due to cell number, while cell size tends to be similar (Rapoport *et al.*, 2004), despite the fact that fruit growth, from the ovary to the mature fruit, is due in greater part to cell expansion than to cell division (Rapoport *et al.*, 2004; Hammami *et al.*, 2011). In fact, the mature fruit has about 8.5 times the cell number compared to the ovary, while cell size is about 250 times greater (Rapoport *et al.*, 2004). Fruit weight is about 2000 (from 1000 to 4000) times greater than ovary weight at bloom (Rosati *et*  *al.*, 2009). However, from the ovary to the mature fruit, the mesocarp grows much more than the endocarp (Rosati *et al.*, 2012), probably due to its longer growth period (i.e. up to fruit maturity) compared to the endocarp, which stops growing about eight weeks after bloom (Hammami *et al.*, 2011). However, both tissues grow in strict proportion to their initial cell number in the ovary. The greater growth of the mesocarp is related to its initial (i.e. in the ovary at bloom) greater number of cells of smaller size (Rosati *et al.*, 2012). These results agree with the hypothesis that cell number, rather than tissue mass, is related to the sink strength of an organ (Ho, 1992) even though this might be not always the case (Marcelis, 1996), because cell number is only one factor determining sink strength (Gillaspy *et al.*, 1993).

72 Olive Germplasm – The Olive Cultivation, Table Olive and Olive Oil Industry in Italy

**Figure 1.** Flower size differs among olive cultivars: Nocellara del Belice, a large-fruited cultivar, on the

Fruit size is important commercially and the study of factors affecting it are of great scientific interest. Fruit size depends mostly on the size and number of fruit cells, though intercellular space may be also important (Bertin *et al.*, 2002; Corelli-Grappardelli, 2004). In olive, fruit size differs greatly among cultivars (Barranco, 1999). Both the endocarp and mesocarp contribute to final fruit size differences among cultivars (Hammami *et al.*, 2011).

Fuit size differences across cultivars are mostly due to cell number, while cell size tends to be similar (Rapoport *et al.*, 2004), despite the fact that fruit growth, from the ovary to the mature fruit, is due in greater part to cell expansion than to cell division (Rapoport *et al.*, 2004; Hammami *et al.*, 2011). In fact, the mature fruit has about 8.5 times the cell number compared to the ovary, while cell size is about 250 times greater (Rapoport *et al.*, 2004). Fruit weight is about 2000 (from 1000 to 4000) times greater than ovary weight at bloom (Rosati *et* 

left, and Koroneiki, a small-fruited cultivar, on the right.

**2. Ovary vs. fruit characteristics** 

Mature fruit size correlates with flower and, particularly, ovary size at bloom (Rosati *et al.*, 2009). This is true for both the mesocarp and the endocarp independently, though, as stated above, the mesocarp grows proportionally more: the endocarp volume in the fruit is about 4000 times greater than its volume in the ovary, while the same ratio is about 800 for the endocarp (Rosati *et al.*, 2012).

Both the endocarp and mesocarp contribute to the ovary size differences among and within cultivars and even the locular space is proportional to ovary size (Rosati *et al.*, 2012). Similarly, proportionality among the ovary and other flower parts was found both within trees (Cuevas & Polito, 2004) and between cultivars (Rosati *et al.*, 2009). Ovary size depends mostly on cell number rather than size, both across and within cultivars (Rosati *et al.*, 2011a). This is true both for the mesocarp and the endocarp, suggesting that similar mechanisms regulate cell division in different tissues. Cell size does differ among cultivar, but does not correlate with ovary size. Cell size differs greatly among tissues with bigger but fewer cells in the endocarp compared to the mesocarp, suggesting that cell growth patterns differ among tissues (Rosati *et al.*, 2011a). In fact, cell size in the ovary correlates with tissue relative growth (i.e. from bloom to fruit maturity), both across tissues and cultivars, suggesting that cell size at bloom reflects the stage of growth of a given tissue: the larger the cells, the less growth remains to be performed (Rosati *et al.*, 2012). This agrees with the observation that fruit growth is mostly achieved by cell expansion as mentioned above. The larger cell size of the endocarp at bloom suggests that this tissue is at a more advanced stage of growth, compared with the mesocarp and thus, its remaining growth, relative to its size at bloom, is less, at least in terms of cell expansion. This would also explain why the endocarp stops growing earlier than the mesocarp. It is possible that the endocarp might be in a more advanced stage of development at bloom, since it is inside this tissue that vital functions (e.g. fertilisation) take place soon after bloom and thus the endocarp needs relatively more differentiated cells. The mesocarp, on the contrary, does not perform particular activities at bloom, hence, it does not need advanced cell differentiation in this period.

In conclusion, genetic differences in fruit size, among olive cultivars, appear to arise from differences in cell division patterns occurring in the ovary (and probably in the whole flower) before bloom. Similar results have been found in other species where genes coding for cell division before bloom have been found to be responsible for differences in final fruit

size. The final fruit size, aside from genetic control, is also related to environmental and endogenous plant conditions that allow the genetic potential growth to be achieved to a varying degree. Analysis the genomics, or of environmental factors affecting fruit size is beyond the scope of this review.

Floral Biology: Implications for Fruit Characteristics and Yield 75

flowers/fruits or that decrease available resources, usually result in increased abortion and decreased fruit set. Among such conditions are: N deficiency, foliar diseases and low leaf-tobud ratio, (Petri, 1920; Morettini, 1951; Uriu, 1953, 1959; Fernandez-Escobar *et al.*, 2008); water stress (Melis, 1923; Brooks, 1948; García Gálvez, 2005); insufficient light (Bottari, 1951; Dimassi *et al.*, 1999); adverse climactic conditions and high yield in previous year (Rallo *et al.*, 1981; Rapoport & Rallo, 1991; Lavee, 1996; Cuevas *et al.*, 1994); abundant flowering in current year (Reale *et al.*, 2006); and unfavorable inflorescence position in the canopy (Cuevas & Polito, 2004; Seifi *et al.*, 2008). Aborted flowers do not contain starch, suggesting a link between nutrient availability and pistil abortion (Reale *et al.*, 2009). Removing part of

the inflorescences reduces drastically the ovary abortion in olive (Seifi *et al.*, 2008).

fruit set.

**4. Fruit set** 

flowers may drop.

On the other hand, pistil abortion is known to be under genetic control, varying with the cultivar (Campbell, 1911; Morettini, 1939, 1951; Magherini, 1971; Lavee, 1996; Lavee *et al.*, 2002). In a recent paper it has been demonstrated that even the genetic component can be interpreted with the competition theory (Rosati *et al.*, 2011b). In fact, as described above, fruit size differences are related to ovary size differences among cultivars. Large-fruited cultivars tend to have bigger flowers, not just ovaries (Rosati *et al.*, 2009). This implies greater energetic costs for the same number of flowers, and thus greater pistil abortion, which in fact tends to be higher in larger-fruited cultivars (Morettini, 1939; Magherini, 1971; Acebedo, 2000; Rosati *et al.*, 2011b). In the previous paragraph we have seen that greater ovary size in large-fruited cultivars results from greater cell number (not greater cell size) and cell number appears to be a key factor in determining the sink strength of an organ (Marcelis, 1996). Larger ovaries with more cells, therefore, might have a greater ability to compete for resources than smaller ovaries, determining both greater abortion and lower

As mentioned above, fruit set is very low in olive (Hartmann, 1950). It is often believed that positively affecting it (i.e. reducing it) would lead to greater yields. However, many studies show that when flower numbers are artificially reduced, fruit set increases proportionally, leading to similar numbers of set fruits (Suarez *et al.*, 1984, Rallo & Fernandez-Escobar, 1985, Lavee *et al.*, 1996, 1999). This is interpreted as a tendency of the olive tree to set a fixed mass of fruits, that is independent of flower number. This fixed mass is referred to as the fruiting potential (Lavee *et al.*, 1996), and depends on the genotype and environmental factors. Hence, as soon as fruit set reaches such potential, based on tree reserves, the rest of the

Competition for resources among flowers in olive is reported in many studies (Suarez *et al.*, 1984; Cuevas *et al.*, 1994; Lavee *et al.*, 1999; Seifi *et al.*, 2008; Rapoport & Rallo, 1991; Cuevas *et al.*, 1995). Rugini and Pannelli (1993) showed that fruit set increases when shoot development is mechanically or chemically slowed down, further supporting the hypothesis that competition drives fruit set. Given that resource availability affects fruit set, largefruited cultivars, having larger ovaries/flowers (Rosati *et al.*, 2009), which need more resources, set fewer fruits, though total fruit mass is similar (Rosati *et al.*, 2010). This

**Figure 2.** Fruit size differing in olive cultivars: Nocellara del Belice, a large-fruited cultivar, on the right, and Koroneiki, a small-fruited cultivar, on the left at the same date during the fruit growth cycle. The difference was already present among the ovaries at bloom.
