Olive is a traditionally important crop grown extensively in the Mediterranean basin. Recent modernization of olive cultivation has introduced and promoted densely planted orchards that are irrigated via systems that can also be used for nutrient application (Fernández-Escobar et al., 2006; Fontanazza, 1993; Tombesi, 2006). Plant growth, fruit production, and oil yield and quality are all expected to be influenced by levels of available nutrients, and manipulation of these levels in irrigation water is necessary to optimize oil production and economic returns.
Flowering and fruit set are the main processes influencing the productivity of fruit trees and are particularly important for olive, where biannual bearing is acutely experienced and where there is an apparently delicate relationship between vegetative and reproductive stages of growth (Lavee, 2006). Inflorescence in olive develops from buds borne on shoots grown in the previous year, a process that starts at the beginning of the summer (Fabbri and Benelli, 2000; Fernández-Escobar et al., 1992). Olive trees possess hermaphrodite and staminate (“male”) flowers. The relative proportion of these two flower types can vary greatly, as a function of cultivar, developmental, and fruiting history, and specific environmental conditions (Lavee et al., 1996). In the northern hemisphere, floral differentiation is evident by March (Hartmann, 1951) and anthesis occurs by April and May. Shortly after anthesis, massive abscission of flowers and fruitlets occurs (Rallo and Fernández-Escobar, 1985; Rapoport and Rallo, 1991a, 1991b). The remaining fruit typically manage to persist on the trees until ripening, which takes place during the fall. At full bloom, some 500,000 flowers are present on a mature olive tree. Commercial yield is achieved if 1% or 2% of those flowers remain as developing fruit (Martin et al., 2005).
Substantial amounts of nutrients are lost from olive trees as a result of fruit removal, annual pruning of leaves and wood, and natural leaf drop. Removed nutrients must be replaced and, where natural levels in the soil are insufficient, appropriate fertilization is necessary to supply the minerals for new growth and for the following year's yield. The current study concentrated on the importance of the macronutrients nitrogen, phosphorus, and potassium and their specific roles in flowering and fruit set of olive.
Olive leaves and stems represent storage organs for N and release it in response to the metabolic demands of developing reproductive and vegetative organs (Fernández-Escobar et al., 2004; Klein and Weinbaum, 1984). Under N deficiency, fruit set, yield, and shoot growth are negatively influenced (Freeman et al., 2005). In experiments in the 1950s in the United States, N fertilization increased fruit set in rain-fed olives grown in nutrient-poor soil (Hartmann, 1958). In Italy, N fertilization was also reported to increase fruit set in olive (Cimato et al., 1990). N was found to increase the proportion of hermaphrodite flowers, and a N concentration of less than 1% in leaves led to the formation of staminate flowers and therefore decreased the potential level of fruit set (Therios, 2006). Therios (2006) also reported that N deficiency reduces the number of flowers. Further evidence of the importance of N to olive productivity was provided by Chatzissavvidis et al. (2004), who found a correlation between a reduction in leaf N and a reduced number of flowers per inflorescence, and by Lombardo and Briccoli-Bati (1990), who showed a significant increase in flower-bud differentiation after fertilization with N of olives planted in poor soils.
Information relating P fertilization to reproductive and vegetative growth and function in olives is limited. Generally, P fertilization is not recommended or practiced in rain-fed olive orchards (Therios, 2006), and there is very little documentation of P deficiencies in this crop (Freeman et al., 2005; López Villalta, 1996). However, it is anticipated that under intensive irrigation management, P supply in many soils will eventually be diminished and that application of P will become beneficial.
Potassium fertilization is essential in olive, particularly because greater than 60% of the plant's K is located in the fruit and is removed annually with its harvest (López Villalta, 1996). Soil application of a large quantity of fertilizer containing K was shown to be responsible for major yield increases in olive orchards that had previously been seriously K deficient (Hartmann et al., 1986).
Some micronutrients have also been found to play roles in plant productivity. In olives, boron appears to be particularly important. Deficient B has been demonstrated to increase the percentage of imperfect flowers and to decrease fruit set (Perica et al., 2001).
Macronutrient mineral status influences the productive stages of olive growth, including flowering and fruit set, directly or indirectly, through its effects on other physiological processes (Fabbri and Benelli, 2000). Indeed, most studies found in the literature indicate a positive response of olive productivity to increased mineral status (Ben Rouina et al., 2002; Talaie and Taheri, 2001). However, the data supporting this have more often than not been collected under rain-fed conditions where the nutrients were applied to the soil before the rainy season or were occasionally sprayed onto the foliage. Little is known regarding the effects of nutrients supplied intensively via the irrigation water throughout the growing season, as is becoming common practice in intensively managed olive orchards. Therefore, the objective of the present work was to study the independent effects of N, P, and K concentrations in the irrigation solution on flowering and fruit set in olive trees.
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