The olive (Olea europaea L.) is an evergreen tree that has been cultivated from ancient times. It occupies a surface of around 10 million hectares all over the world, of which only around 10% is irrigated. It is a species adapted to adverse conditions, grown mainly on dryland areas and calcareous soils around the Mediterranean Basin. Under these conditions, potassium represents the main nutritional problem of olive trees (Restrepo-Diaz et al., 2008). Little is known on P nutrition in the olive, since no growth response to P has been observed under field conditions (Fernández-Escobar, 2008a; Hartmann et al., 1966).
However, P is a plant macronutrient that plays an important role in many plant physiological processes. It is a structural constituent of macromolecules such as nucleic acids, and forms part of nucleoproteins, phospholipids, and a large number of enzymes involved in energy processes. These are directly correlated with the development of the plant and, particularly, its root system (Rausch and Bucher, 2002; Schachtman et al., 1998). Insufficient availability or extremely high levels of P significantly affect plant growth and development (Zeng et al., 2014). Furthermore, the intricate mechanisms involved in maintaining phosphate (Pi) homeostasis reflect the complexity of Pi acquisition and translocation in plants (Raghothama, 1999).
Phosphorus is a limiting factor for crop yield on more than 30% of the world’s arable land (Vance et al., 2003), and its concentration in soils is low compared with other macronutrients (Bieleski, 1973). More than 80% of P becomes immobile and unavailable for plant uptake because of adsorption, precipitation, or conversion to the organic form (Holford, 1997). In addition, the movement of P in soils takes place mainly by diffusion and the rate of this process is slow (Schachtman et al., 1998). Therefore, applications of chemical P fertilizers are needed to improve crop growth and yield in many agricultural systems (Shen et al., 2011). Because of the limited and nonrenewable P resources for fertilizer production, P fertilizer applications will be restricted within the next 60–90 years. Consequently, the increase in crop P efficiency is currently receiving more attention (Thornton et al., 2014).
Phosphorus uptake, cycling, and use efficiency has been investigated intensively with annual plants, but little is known in perennial, woody plants (Rennenberg and Herschbach, 2013). In deciduous trees, P is remobilized from senescing leaves in autumn and stored in other tissues for reuse in the following spring (Barrelet et al., 2006; Eschrich et al., 1988; Kurita et al., 2014). Other strategies for P acquisition and utilization by woody plants include mycorrhizal symbioses (Smith and Read, 2008), and downregulation of P uptake (de Campos et al., 2013). Since P is easily reused in woody plants, severe P deficiency to produce symptoms in leaves is rare in mature trees (Shear and Faust, 1980). In olive orchards is also rare to find leaf P concentrations reaching the critical value for deficiency (Fernández-Escobar, 2008a). Also, P removed in olive trees by yield and pruning is very low (6.9 kg·ha−1·year−1) compared with other macronutrients (Fernández-Escobar et al., 2015). This may explain the lack of response to P fertilization of olive trees growing under field conditions (Hartmann et al., 1966). Nevertheless, P is a nutrient normally applied through fertilization in olive orchards (Fernández-Escobar, 2008b).
Neither P deficiency nor P toxicity has been observed in olive grown under field conditions. However, it might be interesting to know the symptomatology associated with these nutritional disorders and the nutritional status of the olive to which these symptoms occur. Thus, the objective of this work was to study the response of young olive plants to P application, to determine the critical values which may cause deficiency or toxicity symptoms in these plants to a better understanding of P nutrition, and also to determine phosphorus uptake efficiency (PUE).
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