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235 Nitrogen is the element that generally most limits plant productivity in natural and agricultural ecosystems. The response to applied nitrogen mainly depends on the potential crop productivity and on the natural soil nitrogen availability. In Trás-os-Montes, Portugal, most soils display low or even very low phosphorus levels when determined by soil analysis. However, when the phosphorus status of the orchards is assessed by leaf analysis, the results are often found within the "adequate" range. The region’s soils often display high levels of potassium. However, there have been observed situations of visible symptoms of potassium deficiency in olive trees and in some situations already diagnosed by leaf analyzes. In the region, there is also a natural limitation of boron in the soil, being frequent the occurrence of boron deficiency symptoms in dicotyledonous species, such as in olive. Boron application in olive groves is currently a regular practice, although few studies with this nutrient had been done. Thus, the general goal of this work is to collect information that could help laboratories and producers to improve the fertilization programs for the macronutrients nitrogen, phosphorus and potassium and also for boron, the four elements that are most likely to cause nutritional disorders in the olive groves of the region of Trás-os-Montes. To study the olive tree response to the application of nitrogen, phosphorus, potassium or boron, two field trials (with the cultivar ‘Cobrançosa’) and seven pot experiments were installed. The first field trial (Field1) was established in a three-year-old olive grove in which a basal fertilization program with nitrogen, phosphorus, potassium and boron was established as the control treatment. The other four treatments consisted on removing each one of the nutrients. The fertilizers were applied manually at early April localized close to the trunk of the trees. Phosphorus and potassium were applied in areas of 16 (4 m x 4 m) m2 (2 m from the trunk for each quadrant) and nitrogen and boron in areas of 4 m2 (1 m from the trunk for each quadrant). Nitrogen, phosphorus, potassium and boron were applied, respectively, at the rates of 48, 70, 133 and 1.2 g tree-1 year-1. The fertilizers used were ammonium nitrate (34.5 %N), superphosphate (18 % P2O5), potassium chloride (60 % K2O) and borax (11 % B). The second field experiment (Field2) was established after of planting young olive trees specifically for this work. In this experiment, nitrogen, phosphorus, potassium and boron were applied annually, respectively, at the rates of 200, 175, 332 and 6 g per experimental unit (10 trees). The study of nitrogen included a pot experiment (PotN) with four nitrogen rates (0, 0.4, 0.8 and 1.6 g pot-1 year-1). Phosphorus was study from a first experiment (PotP1) in which four phosphorus rates (0, 0.35, 0.70 and 1.05 g pot-1 year-1) were used and from a second experiment (PotP2) in which two rates of phosphorus (0 and 1.05 g pot-1 year-1) were included in a randomized block design where four different soils act as blocks. Potassium was studied from a first pot experiment (PotK1) which includes three potassium rates (0, 0.66 and 1.33 g pot-1 year-1) and from a second experiment (PotK2) arranged as a factorial, with two potassium rates (0 and 0.66 kg pot-1 year-1), two water regimes (normal and stress) and two cultivars (‘Cobrançosa’ and ‘Arbequina’). Boron was also studied from two experiments, the first (PotB1) consisting of the application of boron to the soil [annual application of 0.29 g of borax (11 % B)] and as foliar spray [annual application of 0.04 mL of Tradebor 11 % B)] and the second (PotB2) consisting of the application of boron as a foliar spray [annual application of 0.04 mL of Tradebor (11 % B)] in only a few tagged branches of the plant. This second experiment involved the cultivars 'Cobrançosa' and ‘Arbequina’. Depending on the experiment, several determinations were performed, regarding the biometry of fruits, crop nutritional status, soil properties and olive yield in the first field trial. Whenever considered relevant, chlorophyll fluorescence and transient fluorescence based on the OJIP test were evaluated using the OS-30p+ fluorometer, as well as an index of sclerophylly, leaf tissue density (D), and two water status indices of the plant (relative water content, CRA, and saturation water content, CAS). The results of the first field trial, in which was possible to assess olive yield, showed a significant increases in olive yield only in one of the three years of study, probably because the reduced nitrogen removal by the young trees. In the Field2, nitrogen application significantly increased dry matter yield, leaf nitrogen concentration and nitrogen removal. The pot experiment showed a typical nitrogen response curve, where the production of biomass increases while the availability of nitrogen limits plant growth followed by a stabilization at a plateau or even decreasing when nitrogen is supplied at a high or excessive rate. The increase in nitrogen availability leads to an increase in the ratios between aerial biomass and roots and between leaves and stems. The results also showed that the leaves are a priority sink for nitrogen due to their content of chlorophyll. They also suggest that nitrogen could be taken up by the plant in luxury consumption. The experimental data stressed the difficulty in obtaining response in olive tree growth and yield to phosphorus application. However, in the PotP2, where four different soils were used, some of them with a very low pH, phosphorus application significantly increased total dry matter yield, phosphorus concentration in the tissues, and improved the photosynthetic activity of the leaves. Phosphorus enhanced root growth, showing the ability to accumulate in the root system when it is available in the soil, seeming to be able to buffer the supply of phosphorus to the shoot. In the field trials, potassium did not have a significant effect on tree’s growth and yield. Other parameters related to chlorophyll a fluorescence and leaf water status, also did not significantly vary with potassium application, which apparently reduces the importance of potassium fertilization in these soils. In pot experiments potassium application increased the shoot/root ratio and also the potassium concentration in the root proportionally more than that in the shoot, indicating that shoot is a priority sink for potassium and also that potassium accumulated in the root can act as a reserve for the plant when potassium uptake is reduced. Water stress reduced dry matter yield and potassium concentration in the roots. ‘Arbequina’ showed greater sensitivity to water stress than ‘Cobrançosa’, but produced more dry biomass under well irrigated conditions, possibly because ‘Arbequina’ is an early maturing crop. Boron application increased the nutrient concentration in all tissues but did not cause any relevant effect on the agronomic performance of the plant including olive yield. The results indicate that different sufficiency ranges should be used for samples collected in the summer, at endocarp sclerefication, and in the winter, in the resting period of the olive tree. In PotB1 tissue boron concentrations were significantly higher in the plants from the pots receiving boron to the soil in comparison to those receiving foliar boron, whereas the later showed significantly higher tissue boron concentrations than the plans of the control treatment. The results of PotB2 suggest that boron may have some mobility in the phloem, since after had been applied to the shoot, boron concentration in the root increased. However, the results also suggest that boron mobility in olive may be cultivar dependent. The younger leaves of ‘Arbequina’ showed lower boron concentration than older leaves after the later had been sprayed with boron, an aspect not observed with ‘Cobrançosa’.