**4. Fruit set**

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

beyond the scope of this review.

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

**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

Ovary or pistil abortion indicates the presence of flowers with absent or only partly formed and non-viable ovaries: that is, ovaries incapable of becoming fruits and destined to drop. Flowers with aborted ovaries are often called staminate flowers since only the male organs are complete and functional. Normal flowers with both male and female organs complete and functional are called hermaphrodite flowers. Ovary abortion varies greatly with year, cultivar, individual tree, branch and shoot, and even among and within inflorescences (Morettini, 1939; Bottari, 1951; Badr & Hartmann, 1971; Fabbri *et al.*, 2004; Martin & Sibbett, 2005). Ovary abortion occurs early during flower development, (Pirotta & De Pergola, 1913), mostly 30-40 days before bloom (Uriu, 1959; Cuevas *et al.*, 1999; Reale

Ovary abortion in olive appears to results mostly from competition among flowers (and ovaries, the future fruits) for resources, which are insufficient for all flowers to develop, given the redundant flowering. The onset of this competition occurs very early and affects both pistil abortion and fruit set, as we will see later (Hartmann, 1950; Uriu, 1959; Cuevas *et al.*, 1994; Perica *et al.*, 2001; Levin & Lavee, 2005). Conditions that affect competition among

difference was already present among the ovaries at bloom.

**3. Ovary (or pistil) abortion** 

*et al.*, 2006).

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 flowers may drop.

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

suggests compensation between fruit number and size. The genetic effect on fruit set, appears thus to result from different degrees of competition for resources among ovary/flowers of different sizes. This would explain why small-fruited cultivars like Arbequina and Arbosana tend to produce several fruits per inflorescence, while table-olive (i.e. large-fruited) cultivars typically produce only one, but bigger fruit. Nor can it be argued that the small size in small-fruited cultivars is the result of a higher fruit set and consequent source limitations. In fact, aside from the fact that the fruit of these small-fruited cultivars is smaller already at bloom (i.e. the ovary), well before fruit set is decided, thinning fruits to reduce competition increases fruit size minimally, but does not result in fruit size comparable to that of large-fruited cultivars (Rosati *et al.*, 2010). This suggests that differences in fruit size are mainly of genetic origin. Clearly, the ultimate fruit size depends also on resource availability, and varies up to more than 100% within the same cultivar, but among cultivars fruit size may vary much more than that (Barranco, 1999), up to 600% (Rosati *et al.*, 2009). Hence, adjusting fruit load across cultivars with extremely different fruit size needs a wider compensation mechanism than just adjusting fruit size: this mechanism appears to be the adjustment of fruit set.

Floral Biology: Implications for Fruit Characteristics and Yield 77

To increase olive productivity, therefore, it might be possible to breed plants that invest less in male fitness (reduced flowering), thus saving resources, which can be spend in setting and maturing more fruits. This appears reasonable since the energetic cost of flowering is

We have seen how competition for available resources among developing flowers and fruits plays a continuous role during the whole developmental cycle of the reproductive organs (Lavee *et al.*, 1999). Hence, pistil abortion first, and fruit set afterwards, appear to be part of a single mechanism that adjusts maternal investments to available resources. Even the genetic component affecting fruit size (i.e. cultivar differences) may be explained with the competition theory, based on the related differences in flower and ovary size (which correlates with fruit size), implying different energetic costs for the development of one fruit. In the absence of catastrophic events that might impair production (e.g. lack of pollination, severe drought or nutrient deficiency, etc.), the olive tree appears to produce fruits according to its potential, independent of the amount of flowers produced. To achieve this potential, the tree uses the different compensatory mechanisms available at any given developmental stage (i.e. pistil abortion, fruit drop, fruit size). Hence, studies reporting positive correlations between abundance of flowering (or pollen in the air) and yield (Moriondo *et al.*, 2001; Fornaciari *et al.*, 2002, Galán *et al.*, 2004), would imply not a causal relationship but simply correlated phenomena: bloom and yield both represent an expression of the tree yield potential. The redundant flowering in olive, may serve the purpose of increasing the mail and the overall fitness, which is advantageous from an evolutionary point of view, but wasteful from an agronomical perspective. For the olive grower, selecting for reduced mail fitness (i.e. fewer flowers) to benefit the female fitness

*Agricultural Research Council - Olive Growing and Oil Industry Research Centre, Spoleto (PG),* 

Financial support for this study was provided by the Italian Ministry of Agriculture, Food and Forestry Policy through the project GERMOLI "Salvaguardia e valorizzazione del

Acebedo, M.M., Cañete, M.L. & Cuevas, J. (2000). Processes affecting fruit distribution and its quality in the canopy of olive trees, *Advances in Horticultural Science* Vol. 14:169–175.

not negligible in olive (Famiani *et al.*, unpublished data).

**6. Conclusions** 

(i.e. yield) might prove beneficial.

Adolfo Rosati, Silvia Caporali and Andrea Paoletti

GERMoplasma OLIvicolo delle collezioni del CRA-OLI".

**Author details** 

**Acknowledgment**

**7. References** 

*Italy* 

### **5. Redundant flowering, andromonecy and fitness**

As stated above, the olive produces redundant flowers relative to its yield potential and fruit set is very low. We have seen how ovary abortion increases under conditions that increase competition for resources. In fact, ovary abortion is a means to save resources, balancing the number of ovaries to the resources available (Primack & Lloyd, 1980; Bertin, 1982; Stephenson & Bertin, 1983). Plants, like the olive, that abort part of the ovaries in otherwise hermaphrodite flowers are called andromonoecious (about 4000 species are estimated to be andrommonoecious). From an evolutionary point of view, andromonoecy is considered an intermediate step towards dioecy, and allows saving of resources without affecting the total number of flowers and thus the male function and fitness (Vallejo-Marín & Rausher, 2007). Aborting the ovary does not affect pollen production in olive (Cuevas & Polito, 2004). In this species, large-fruited cultivars have greater ovary abortion, but similar number of flowers (Rosati *et al.*, 2010, 2011b), leaving the male function probably unaltered (although differences in pollen production per flower, among cultivars with different fruit/ovary/flower size have not been studied). Leaving the male function (and fitness) unaltered might explain why only the ovary is aborted instead of the whole flower, which would save more resources, given that the flower is several times bigger than the ovary alone (Rosati *et al.*, 2009; Cuevas & Polito, 2004). It would also explain why the flower production is so redundant compared to the fruiting potential. Producing pollen is less expensive than producing fruits and seeds, while still increasing the plant's fitness, making it more convenient to produce more (male) flowers that fruits. Modeling work demonstrates that fitness is maximized, under limiting resources, when many more flowers than fruits are produced (Morgan, 1993). This author finds that the optimal flower/fruit ratio increases for andromonoecious and monoecious species compared to dioecious ones, and increases even more for wind pollinated species, like the olive.

To increase olive productivity, therefore, it might be possible to breed plants that invest less in male fitness (reduced flowering), thus saving resources, which can be spend in setting and maturing more fruits. This appears reasonable since the energetic cost of flowering is not negligible in olive (Famiani *et al.*, unpublished data).
