Due to its present and future strategic value, olive production systems for the production of olive oil have received a strong push in terms of technological innovation in the last twenty years. A higher number of plants per hectare, mechanized harvesting and the use of fertigation are some of the innovations that involve what is called agricultural intensification. The intensification of olive orchard management implies an increase in irrigation and fertilization, especially nitrogen (N), which can lead to a reduction in virgin olive oil’s (VOO1VOO includes Extra Virgin and Virgin Olive Oil categories as are described in the Regulation (EU) No 1308/2013 (EU, 2013).) quality and stability, in particular, a decrease in phenol content. There is a negative relationship between irrigation and phenol content, as different authors have demonstrated in studies with linear irrigations, where the total applied irrigation water linearly changed with the effective crop coefficient (Tovar et al., 2001Tovar MJ, Motilva MJ, Luna M, Girona J, Romero MP. 2001. Analytical characteristics of virgin olive oil from young trees (Arbequina cultivar) growing under linear irrigation strategies. J. Am. Oil Chem. Soc. 78, 843-849. https://doi.org/10.1007/s11746-001-0353-5. ; Berenguer et al., 2006Berenguer MJ, Vossen PM, Grattan SR, Connell JH, Polito VS. 2006. Tree irrigation levels for optimum chemical and sensory properties of olive oil. HortScience 41, 427-432. https://doi.org/10.21273/HORTSCI.41.2.427. ) or deficit irrigations (Gómez-del-Campo, 2013Gómez-del-Campo M. 2013. Summer deficit irrigation in a hedgerow olive orchard cv. Arbequina: Relationship between soil and tree water status, and growth and yield components. Spanish J. Agric. Res. 11, 547-557. https://doi.org/10.5424/sjar/2013112-3360. ; Rufat et al., 2018Rufat J, Romero-Aroca AJ, Arbonés A, Villar JM, Hermoso JF, Pascual M. 2018. Mechanical harvesting and irrigation strategy responses on ‘Arbequina’ olive oil quality. Horttechnology 28, 607-614. https://doi.org/10.21273/HORTTECH04016-18. ), although no differences were found in intermediate irrigation strategies (Sastre et al., 2019Sastre B, Arbones A, Benito A, Rufat J, de Lorenzo C, Pascual M, Villar JM, Perez MA. 2019. Efecto del riego deficitario en un superintensivo de Arbequina sobre la calidad del aceite de oliva virgen, Expoliva, May 2019. ), which linearly increased the total amount of applied water. Berenguer et al. (2006)Berenguer MJ, Vossen PM, Grattan SR, Connell JH, Polito VS. 2006. Tree irrigation levels for optimum chemical and sensory properties of olive oil. HortScience 41, 427-432. https://doi.org/10.21273/HORTSCI.41.2.427. found differences among the most contrasting treatments. Phenols are highly influenced by cultivar and ripening stage (Aguilera et al., 2017Aguilera M, Uceda M, Beltran G. 2017. La calidad del aceite de oliva, in Barranco D, Fernandez-Escobar R, Rallo L (Eds.) El cultivo del olivo, 7th ed, Ediciones Muni-Prensa, Madrid, pp. 839-868. ), and also by other agronomic factors such as harvesting method (Morales-Sillero and García, 2015Morales-Sillero A, García JM. 2015. Impact assessment of mechanical harvest on fruit physiology and consequences on oil physicochemical and sensory quality from ‘Manzanilla de Sevilla’ and ‘Manzanilla Cacereña’ super-high-density hedgerows. A preliminary study. J. Sci. Food Agric. 95, 2445-2453. https://doi.org/10.1002/jsfa.6971. ; Arbonés et al., 2016Arbonés A, Pascual M, Villar JM, Romero A, Rufat J. 2016. Respuesta cualitativa al riego y a la recolección mecanizada en olivo superintensivo (cv. Arbequina). Actas del I Congreso Ibérico de Olivicultura / V Jornadas Nacionales del Grupo de Olivicultura de la Sociedad Española de Ciencias Horticolas (SECH) / VII Simpósio Nacional de Olivicultura da Associação Portuguesa de Horticultura (APH), April 2016, pp. 137. ).
Water availability is important for determining plant response to mineral nutrition (Marschner, 2012Marschner H. 2012. Marschner’s Mineral Nutrition of Higher Plants. 3rd ed, Academic Press. ) at both the irrigation level and N status, which are highly related to the cultivar. At present, the question stated by Zipori et al. (2015)Zipori I, Yermiyahu U, Erel R, Presnov E, Faingold I, Ben-Gal A, Dag A. 2015. The influence of irrigation level on olive tree nutritional status. Irrig. Sci. 33, 277-287. https://doi.org/10.1007/s00271-015-0465-5. should be answered “Can threshold values being used today be applied indiscriminately to all cultivars, especially in intensive, irrigated orchards?”
The influence of ripeness and irrigation on the phenol content in VOOs phenols has been studied in recent years (Aparicio and Harwood, 2013Aparicio R, Harwood J. 2013. Handbook of Olive Oil: Analysis and Properties. Springer, Boston, MA: Springer US. https://doi.org/10.1007/978-1-4614-7777-8 ; Mena et al., 2018Mena C, Gonzalez AZ, Olivero-David R, Perez-Jimenez, MA. 2018. Characterization of ‘Castellana’ Virgin Olive Oils with Regard to Olive Ripening. Horttechnol. 28,48-57. https://doi.org/10.21273/HORTTECH03845-17. ; Vidal et al., 2019Vidal AM, Alcala S, de Torres A, Moya M, Espinola F. 2019. Characterization of Olive Oils from Superintensive Crops with Different Ripening Degree, Irrigation Management, and Cultivar: (Arbequina, Koroneiki, and Arbosana). Eur. J. Lipid Sci. Technol. 121, 1800360. http://doi.org/10.1002/ejlt.201800360. ), although there are few works related to fertilization (Lanza and Ninfali, 2020Lanza B, Ninfali P. 2020. Antioxidants in extra virgin olive oil and table olives: Connections between agriculture and processing for health choices. Antioxidants 9, 41. https://doi.org/10.3390/antiox9010041. ; Zipori et al., 2020Zipori I, Erel R, Yermiyahu U, Ben-Gal A, Dag A. 2020. Sustainable management of olive orchard nutrition: A review. Agriculture-Basel 10, 11. https://doi.org/10.3390/agriculture10010011. ) despite its crucial role in VOO’s quality. Fertilization should be carefully monitored to avoid negative impacts on VOO (Rallo et al., 2018Rallo L, Diez CM, Morales-Sillero A, Miho H, Priego-Capote F, Rallo P. 2018. Quality of olives: A focus on agricultural preharvest factors. Scientia Hortic. 233, 491-509. https://doi.org/10.1016/j.scienta.2017.12.034. ). This work aims to review existing studies and research related to the influence of olive orchard fertilization on the phenols in VOO.
2. NITROGEN, PHOSPHORUS AND POTASSIUM INFLUENCE ON VOO PHENOLS⌅
Despite the fact that most of the studies show that an increase in N fertilization leads to a decrease in the phenol content of the VOOs in a different way, depending on olive orchard management and cultivars, some results are not so clear (Inglese et al., 2002Inglese P, Gullo G, Pace LS. 2002. Fruit growth and olive oil quality in relation to foliar nutrition and time of application. Acta Hortic. 586, 507-509. https://doi.org/10.17660/ActaHortic.2002.586.105. ; Mezghani et al., 2018Mezghani MA, Ayadi M, Attia F, Zouari I, Labidi F, Attia L. 2018. Effects of irrigation and fertigation on tree growth, yield and oil quality of olive cultivars in south tunisia. Acta Hortic. 1199, 255-260. https://doi.org/10.17660/ActaHortic.2018.1199.39. ).
In a long-term trial under rainfed conditions, Fernández-Escobar et al. (2002)Fernández-Escobar R, Sánchez-Zamora MA, Uceda M, Beltran G. 2002. The effect of nitrogen overfertilization on olive tree growth and oil quality. Acta Horticulturae 586, 429-432. https://doi.org/10.17660/ActaHortic.2002.586.88. found that increasing N doses up to 1 kg N·tree-1 resulted in a decrease in total phenols in VOO with no increase in yield or vegetative growth, despite higher values in leaf and fruit N content compared to the control. This occurred regardless of the plantation system (Fernandez-Escobar et al., 2006Fernández-Escobar R, Beltran G, Sánchez-Zamora M, García-Novelo J, Aguilera M, Uceda M. 2006. Olive oil quality decreases with nitrogen over-fertilization. HortScience 41, 215-219. https://doi.org/10.21273/HORTSCI.41.1.215. ). Moreover, foliar applications of B and N in Koroneiki induced a phenol decrease but a foliar algae solution (derived from fronds and strap-like blades of the seaweed Ascophyllum nodosum L. and applied at a concentration of 0.5%) did not (Chouliaras et al., 2009Chouliaras V, Tasioula M, Chatzissavvidis C, Therios I, Tsabolatidou E. 2009. The effects of a seaweed extract in addition to nitrogen and boron fertilization on productivity, fruit maturation, leaf nutritional status and oil quality of the olive (Olea europaea L.) cultivar koroneiki. J. Sci. Food Agric. 89, 984-988. https://doi.org/10.1002/jsfa.3543. ).
A decrease in total phenol content was observed in response to high doses of N and phosphorus (P) while potassium (K) had only a minor effect or no effect at all on irrigated trees (Dag et al., 2009Dag A, Ben-David E, Kerem Z, Ben-Gal A, Erel R, Basheer L, Yermiyahu U. 2009. Olive oil composition as a function of nitrogen, phosphorus and potassium plant nutrition. J. Sci. Food Agric. 89, 1871-1878. https://doi.org/10.1002/jsfa.3664. ; Lanza and Ninfali, 2020Lanza B, Ninfali P. 2020. Antioxidants in extra virgin olive oil and table olives: Connections between agriculture and processing for health choices. Antioxidants 9, 41. https://doi.org/10.3390/antiox9010041. ), while Pascual et al. (2019)Pascual M, Villar JM, Arbonés A, Rufat J. 2019. Nitrogen nutrition diagnosis for olive trees grown in super-intensive cropping systems. J. Plant Nutr. 42, 1803-1817. https://doi.org/10.1080/01904167.2019.1628983. found a significant decrease with K fertilization. Erel et al. (2013)Erel R, Kerem Z, Ben-Gatt A, Dag A, Schwartz A, Zipori I, Basheer L, Yermiyahu U. 2013. Olive (Olea europaea L.) tree nitrogen status is a key factor for olive oil quality. J. Agric. Food Chem. 61, 11261-11272. https://doi.org/10.1021/jf4031585. , who observed a decreased in phenol content as foliar N increased, found the same trend. They also found a weaker relationship between phenol content and fruit N, without finding a correlation between phenol content and fruit P content. In the Barnea cultivar, Dag et al. (2018)Dag A, Erel R, Kerem Z, Ben-Gal A, Stern N, Bustan A, Zipori I, Yermiyahu U. 2018. Effect of nitrogen availability on olive oil quality. VIII International Olive Symposium 1199, 465-469. https://doi.org/10.17660/ActaHortic.2018.1199.74. observed a reduction in total phenol content when high N doses were applied, similar to what Morales-Sillero et al. (2007)Morales-Sillero A, Jimenez R, Fernandez JE, Troncoso A, Beltran G. 2007. Influence of fertigation in ‘manzanilla de sevilla’ olive oil quality. HortScience 42, 1157-1162. https://doi.org/10.21273/HORTSCI.42.5.1157. observed in the Manzanilla de Sevilla cultivar, although significant differences with fruit N content were not found. Phosphorous enhances the accumulation of N in leaves and fruits and thus indirectly increases the nutritional level of N. Total phenol concentration was entirely independent of the leaf or fruit K levels, and therefore the effect of the N on oil properties depended solely on the absolute N content in the leaves or fruits, regardless of the cause. Olive trees can deal with a low soil fertility unless a great deficiency occurs, but an overdose in N, P or K fertilization can lead to a reduction in VOO quality (Tognetti et al., 2008Tognetti R, Morales-Sillero A, D’Andria R, Fernandez JE, Lavini A, Sebastiani L, Troncoso A. 2008. Deficit irrigation and fertigation practices in olive growing: Convergences and divergences in two case studies. Plant Biosystems 142, 138-148. https://doi.org/10.1080/11263500701872879. ).
Arbonés et al. (2017)Arbonés A, Pascual M, Villar JM, Rufat J. 2017. Riego, abonado nitrogenado y calidad de aceite. Revista de Fruticultura 56,120-129. Ed.Tècnica Quatrebcn. also found a negative correlation between total phenols and N foliar content in a trial with N and K fertilization in super-intensive hedgerows of the Arbequina cultivar in Lleida (Spain). Only in ON years (heavy yield) did K application have a positive response, and a N/K interaction was evidenced by the effect of K on increasing leaf N content. Moreover, a high decrease in total phenols with K fertilization was found (Pascual et al., 2019Pascual M, Villar JM, Arbonés A, Rufat J. 2019. Nitrogen nutrition diagnosis for olive trees grown in super-intensive cropping systems. J. Plant Nutr. 42, 1803-1817. https://doi.org/10.1080/01904167.2019.1628983. ). They also established a critical nutrient range for N leaf in the interval of 1.80-1.94% with no effect on VOO yield, while beyond this threshold, phenol content and oxidative stability were compromised. That statement was confirmed by Arbonés et al. (2020)Arbonés A, Sastre B, Perez MÁ, de Lorenzo C, Pascual M, Benito A, Villar JM, Rufat J. 2020. Influence of irrigation and fertilization on the sterol and triterpene dialcohol compositions of virgin olive oil. Grasas Aceites 71, 376. https://doi.org/10.3989/gya.0795191. , with cv Arbequina in two super-intensive orchards in Madrid and Lleida (Spain) with different levels of irrigation and N-K fertilization, finding a good negative linear correlation (R2=0.81) between foliar N and total phenol content in VOO (Figure 1). Beyond 1.9% leaf N, the total phenol content decreased to below 200 mg·kg-1 (expressed as caffeic acid), a value considered to be the threshold above which phenolic compounds exert their nutraceutical effects as antioxidants (European Food Safety Authority Panel on Dietetic Products, Nutrition and Allergies, 2011European Food Safety Authority Panel on Dietetic Products, Nutrition and Allergies. 2011. Scientific opinion on the substantiation of health claims related to polyphenols in olive and protection of LDL particles from oxidative damage (ID 1333, 1638, 1639, 1696, 2865). EFSA J. 9, 2033. ; Gucci et al., 2012Gucci R, Caruso G, Canale A, Loni A, Raspi A, Urbani S, Taticchi A, Esposto S, Servili M. 2012. Qualitative Changes of Olive Oils Obtained from Fruits Damaged by Bactrocera oleae (Rossi). HortScience 47, 301-306. http://doi.org/10.21273/HORTSCI.47.2.301. ). This fact gives rise to a reduction in VOO quality. The relationship between fruit N (pulp plus stone) and total phenol content is quadratic, and it is highly influenced by crop load (Figure 2). Below 7 g·kg-1 fruit N, VOO’s phenol content falls below the 200 mg·kg-1 (expressed as caffeic acid).
Within the phenol family, increasing foliar and fruit N levels gives rise to a decrease in total oil phenols (around 250 mg per N foliar unit), specially in secoroid compounds, which results in lower VOO quality and stability. This negative impact of N on VOO quality is higher in OFF years (low yield). These results differ from those of Centeno et al. (2017)Centeno A, Garcia JM, Gomez-del-Campo M. 2017. Effects of nitrogen fertilization and nitrification inhibitor product on vegetative growth, production and oil quality in ‘Arbequina’ hedgerow and ‘picual’ vase-trained orchards. Grasas Aceites 68, e215. https://doi.org/10.3989/gya.0441171. with cv Arbequina, who observed lower ortho-diphenol (o-diphenols) contents in all N-fertilized treatments regardless of doses or products, whilst total secoiridoids were only reduced in treatments with N inhibitors. In Picholine cultivar under irrigation conditions, Tekaya et al. (2014)Tekaya M, Mechri B, Cheheb H, Attia F, Chraief I, Ayachi M, Boujneh D, Hammami M. 2014. Changes in the profiles of mineral elements, phenols, tocopherols and soluble carbohydrates of olive fruit following foliar nutrient fertilization. LWT-Food Science and Technology 59, 1047-1053. https://doi.org/10.1016/j.lwt.2014.06.027. found a significant decrease in total phenols with urea and micronutrient fertilization. The same group did not find a negative effect of N fertilization on total phenols but they observed a great decrease in o-diphenol content (Tekaya et al., 2013Tekaya M, Mechri B, Bchir A, Attia F, Cheheb H, Daassa M, Hammami A. 2013. Effect of nutrient-based fertilisers of olive trees on olive oil quality. J. Sci. Food Agric. 93, 2045-2052. https://doi.org/10.1002/jsfa.6015. ).
Nitrogen is one of the nutrients that can modulate the activity of different enzymes related to phenol content and metabolism, like phenylalanine ammonium lyase (PAL) and polyphenol oxidase (PPO). In different plants (e.g. basil, tobacco and olive tree), PAL activity, involved in the synthesis of phenols, can be negatively influenced by N fertilizacion due to a competition between phenols and protein in fruit (Ruiz et al., 1998Ruiz JM, Bretones G, Baghour M, Ragala L, Belakbir A, Romero L. 1998. Relationship between boron and phenolic metabolism in tobacco leaves. Phytochem. 48, 269-272. https://doi.org/10.1016/S0031-9422(97)01132-1. ; Fernández-Escobar, 2011 Fernández-Escobar R. 2011. Use and abuse of nitrogen in olive fertilization. International Symposium on Olive Irrigation and Oil Quality 888, 249-257. https://doi.org/10.17660/ActaHortic.2011.888.28. ; Jakovljević et al., 2019Jakovljević D, Topuzović M, Stanković M. 2019. Nutrient limitation as a tool for the induction of secondary metabolites with antioxidant activity in basil cultivars. Ind. Crops Prod. 138, 111462. https://doi.org/10.1016/j.indcrop.2019.06.025. ). Furthermore, PPO activity increases as foliar and fruit N contents increase, with a negative effect on VOO stability due to changes in its fatty acid profile (Pascual et al., 2016Pascual M, Villar JM, Arbonés A, Rufat J. 2016. Fertilización nitrogenada, actividad enzimática y calidad de producto en melocotonero y olivo. Acta Hortic. 75, ed. SECH, 16-18 October 2016, pp. 54. ).
The synthesis of phenolic compounds is stimulated in response to different plant stresses (Fernández 2014Fernández JE. 2014. Understanding olive adaptation to abiotic stresses as a tool to increase crop performance. Environ. Exp. Bot. 103, 158-179. https://doi.org/10.1016/j.envexpbot.2013.12.003.; Rallo et al., 2018Rallo L, Diez CM, Morales-Sillero A, Miho H, Priego-Capote F, Rallo P. 2018. Quality of olives: A focus on agricultural preharvest factors. Scientia Hortic. 233, 491-509. https://doi.org/10.1016/j.scienta.2017.12.034. ). Therefore, in intensive and super-intensive olive orchards, from the point of view of the quality of the VOO, it should be appropiate to exert a certain level of stress, but not too high so as to reduce crop load.
3. OTHER NUTRIENTS THAT INFLUENCE VOO PHENOLS⌅
A decrease in total phenols and o-diphenols has been observed after foliar fertilization with boron (B), magnesium (Mg), manganese (Mn) and sulfur (S), due to the B action (Tekaya et al., 2013Tekaya M, Mechri B, Bchir A, Attia F, Cheheb H, Daassa M, Hammami A. 2013. Effect of nutrient-based fertilisers of olive trees on olive oil quality. J. Sci. Food Agric. 93, 2045-2052. https://doi.org/10.1002/jsfa.6015. ). When the content of B in the leaves is low, phenolic biosynthesis increases, but when the level of boron is adequate, the PAL activity is depressed and phenol biosynthesis is limited (Ruiz et al., 1998Ruiz JM, Bretones G, Baghour M, Ragala L, Belakbir A, Romero L. 1998. Relationship between boron and phenolic metabolism in tobacco leaves. Phytochem. 48, 269-272. https://doi.org/10.1016/S0031-9422(97)01132-1. ). Under semiarid conditions, Saadati et al. (2013)Saadati S, Moallemi N, Mortazavi S, Seyyednejad S. 2013. Effects of zinc and boron foliar application on soluble carbohydrate and oil contents of three olive cultivars during fruit ripening. Sci. Hortic. 164, 30-34. https://doi.org/10.1016/j.scienta.2013.08.033. did not find statistical differences in total phenols for different cultivars after applying boric acid and zinc sulfate. Increases in VOO oil content and phenolic compounds were observed when cv. Beledy olive trees were treated with Zn in the form of amino acid chelate (Bastam et al., 2020Bastam N, Baninasab B, Mobli M, Goli SAH. 2020. Effects of foliar applications of zinc in the forms of free mineral or amino acid complexed on qualitative characteristics of olive oil. J. Am. Oil Chem. Soc. 98, 173-184. https://doi.org/10.1002/aocs.12443. ). In the Chemlali cultivar, the foliar spraying of amino acids, beet molasses and fish by-products as sources of N, P and K resulted in a decrease in the phenol content, contrary to an application of a formulation based on calcium (Ca), which led to a significant increase in phenolic compound contents (Dabbaghi et al., 2019Dabbaghi O, Tekaya M, Flamini G, Zouari I, El-Gharbi S, M’barki N, Laabidi F, Cheheb H, Attia F, Mezghani MA, Hammami M, Mechri B. 2019. Modification of phenolic compounds and volatile profiles of chemlali variety olive oil in response to foliar biofertilization. J. Am. Oil Chem. Soc. 96, 585-593. https://doi.org/10.1002/aocs.12201. ).
Foliar fertilization with selenium (Se) applied to the Leccino cultivar increased the total Se, alleviated damage caused by drought stress conditions and increased total antioxidant compounds, mainly phenol contents (D’Amato et al., 2018D’Amato R, De Feudis M, Hasuoka PE, Regni L, Pacheco PH, Onofri A, Businelli D, Proietti P. 2018. The selenium supplementation influences olive tree production and oil stability against oxidation and can alleviate the water deficiency effects. Front. Plant Sci. 9, 1191. https://doi.org/10.3389/fpls.2018.01191. ). Silicon (Si), like selenium, is a non-essential element in plant nutrition despite the fact that it plays an important role against drought and other abiotic stresses (Marschner 2012Marschner H. 2012. Marschner’s Mineral Nutrition of Higher Plants. 3rd ed, Academic Press. ). Different studies have found that Si forms a physical barrier and induces the production of phenolic compounds at sites of infection (Fernandez-Escobar, 2019Fernández-Escobar R. 2019. Olive nutritional status and tolerance to biotic and abiotic stresses. Frontiers Plant Sci. 10, 1151. https://doi.org/10.3389/fpls.2019.01151. ). Silicon applying can be done through drip irrigation or the most effective foliar spraying, which can be promising for olive orchards, mainly under rainfed conditions.
Nitrogen (N) over-fertilization leads to a total phenol content decrease, regardless of the olive growing system or cultivar type; phosphorus fertilization seems to have a minor influence, and in the case of potassium (K), the effect is controversial.
Olive yield decreases beyond a threshold of foliar N, for this reason it is essential to establish the proper reference levels, which do not impair olive yield whilst maintaining a sufficient phenol content to maintain good VOO quality and stability. This threshold should be different for each cultivar and olive-growing system.
Phosphorus fertilization seems to have an indirect influence on phenol content, although results on the effect of K differ among authors. There is a lack of field trials with these macro-nutrients near or below the deficiency threshold to clarify the role of K regarding phenol content. New trials with potassium-based formulations could be of great interest, in order to increase total phenols in cultivars with low to medium phenol content.
Fertilization with micronutrients and other non-essential microelements have been scarcely studied. These elements must be considered for future studies, mainly boron and selenium elements, which could produce an increase in total phenols, or silicon, which could protect fruit skin, thus avoiding the loss of phenols during mechanical harvesting.