The olive tree, one of the most emblematic trees of the Mediterranean, has special nutritional requirements.
Among the various nutrients, boron (B) emerges as a very essential trace element with a crucial role in its cultivation.
Physiological role of boron in the olive tree
Effect on flowering and fruiting
Boron in all the reproductive processes of the olive tree, decisively influencing the success of flowering and fruiting:
Pollen grain germination and pollen tube growth:Pollen grain germination and pollen tube growth:
According to Perica et al. (2001), boron is essential for pollen germination and pollen tube development, processes critical for fertilization. Experiments have shown that boron levels below 20 mg/kg in leaves can reduce pollen grain germination by 30-45% [1].According to Perica et al. (2001), boron is essential for pollen grain germination and pollen tube development, processes critical for pollination. Experiments have shown that boron levels below 20 mg/kg in leaves can reduce pollen grain germination by 30-45% [1].
Effect on fruit setting:
Delgado et al. (1994) documented that deficient boron levels are associated with a significant reduction in fruit set in olives, while correction of boron deficiencies increased the fruit set rate by 25-35% in field experiments [2].
Periodicity of production:
Fernández-Escobar et al. (2008) showed that boron sufficiency can help to reduce the intensity of interception, a major problem in olive growing, by improving the stability of production between successive years [3].
Contribution to metabolism and growth
Apart from reproductive processes, boron is also important in the general metabolism and growth of the olive tree:
Cell walls and tissue structure:
Brown et al. (2002) documented the role of boron in cell wall stabilization through its association with components such as pectins. Boron deficiency leads to cell wall disorganization and consequently abnormal tissue growth [4].Brown et al. (2002) documented the role of boron in cell wall stabilization through its association with components such as pectins. Boron deficiency leads to cell wall disorganization and consequently abnormal tissue growth [4].
Transport of sugars:
Research by Hu and Brown (1997) showed that boron facilitates the transport of sugars across membranes, thus affecting the overall metabolic activity and energy economy of the plant [5].
Development of the root system:
Retalis and Vachamidis (2007) demonstrated in a field experiment that sufficient boron supply led to an increase in the root mass of olive trees by 15-20%, improving their ability to absorb water and nutrients [6].
Symptoms and effects of boron deficiency
Boron deficiency in the olive tree manifests itself with characteristic symptoms affecting different parts of the tree:
Symptoms on leaves and shoots
Chlorosis and leaf deformation:
According to Vossen et al. (2005), the leaves show characteristic chlorosis at the tips, while in advanced stages necrosis and deformation occur. The leaf arrangement takes on a ‘rosette’ shape due to a reduction in the length of the intergonate spaces [7].
Reduced shoot growth:
Chatzissavvidis and Therios (2010) observed a 40-55% reduction in vegetative growth under conditions of severe boron deficiency, with characteristic symptoms of terminal bud necrosis and the appearance of multiple lateral shoots, giving the tree a bushy appearance [8].
Symptoms on the wrists
“Monkey face.”
Therios and Dimassi (2004) described the characteristic deformation of the fruit known as “monkey face”, with abnormal growth and formation of dents on the surface, which dramatically reduce the commercial value of the product [9].
Reduction in size and quality:
The research of Tsadilas and Chartzoulakis (1999) showed that boron deficiency leads to a 15-30% reduction in fruit size and negatively affects the oil content of the fruit as well as the organoleptic characteristics of olive oil [10].
Factors affecting boron availability
The availability of boron in the soil and its uptake by the olive tree are influenced by several factors:
Territorial Factors
soil pH:
Gupta et al. (1985) documented that boron availability decreases dramatically in soils with pH > 7. Specifically, for every 1 point increase in pH above 7, boron availability decreases by about 50% [11].
Organic matter:
Research by Goldberg et al. (2000) showed that organic matter is an important source of boron in soil, with soils rich in organic matter (>2%) having 30-45% higher availability [12].
Soil structure:
Nable et al. (1997) showed that sandy soils are more prone to boron leaching, while clay soils can retain larger amounts, but often with reduced availability due to strong adsorption [13].
Climate Factors
Humidity:
Peryea et al. (2003) observed that prolonged drought reduces boron availability due to reduced movement in the soil solution, while heavy rainfall can lead to leaching of the element, particularly in sandy soils [14].
Temperature:
According to Dell et al. (1997), high temperatures increase the boron requirements of plants due to more intense metabolism and faster growth [15].
Application of boron – Proficiency levels
Proper boron management requires an accurate diagnosis of the nutritional status of the trees:
Leaf diagnostics:
Fernández-Escobar et al. (2009) identified optimal levels of boron in olive leaves between 19-150 mg/kg dry matter for samples collected in July. Levels below 14 mg/kg indicate food deficiency, while levels above 185 mg/kg may lead to toxicity [16].
Soil analysis:
Tsadilas et al. (2005) suggest a limit value of 0.5 mg/kg of extractable boron (hot water method) for most soils where olive trees are grown [17].
Application methods
Terrestrial Application
Dosage and frequency:
Soyergin et al. (2010) recommend the application of 100-150 g borax (11% B) per tree for young trees and 200-300 g for fully grown trees, with a frequency of application every 2-3 years depending on the results of the analyses [18]
.
Application time:
According to Fernández-Escobar et al. (2018), the most suitable period for soil boron application is late winter to early spring to ensure sufficient availability during the critical flowering period [19].
Distribution on the ground:
Wojcik and Wojcik (2003) suggest application in a zone extending from the trunk to the crown projection, with light incorporation to avoid losses [20].
Interfacial application
Pros:
Camacho-Cristóbal et al. (2002) documented that transhumance offers a faster response, requires smaller amounts of fertilizer and is particularly effective in alkaline soils where soil boron availability is limited [21].
Gatherings and time:
Perica et al. (2002) recommend spraying with a 0.2-0.5% borax or 0.1-0.2% boric acid solution at two critical periods: (a) just before flowering and (b) during the kernel hardening stage. For severe trophic deficiencies, 2-3 additional applications may be required [22].
Combination with other nutrients:
Research by Reickenberg and Pritts (1996) showed that the combined application of boron with calcium and potassium can enhance the absorption and efficacy of boron, while improving fruit quality [23].
Types of Fertilizers Borium
Borax (Na₂B₄O₇-10H₂O):
It contains about 11% boron and is the most widely used source of boron for soil applications. Sartain and Obrezja (2010) highlighted its relatively slow solubility, which ensures prolonged availability [24].
Boric acid (H₃BO₃):
With 17% boron content, it is suitable for trans-well applications due to its high solubility and absorption. Liakopoulos et al. (2005) showed that boric acid is absorbed from olive leaves 20-30% faster than borax [25].
Chemical forms of boron:
Recent research by Guerreiro et al. (2017) suggests that chelated forms of boron show increased efficacy with reduced risk of toxicity, particularly for transwave applications in environments with large temperature and humidity fluctuations [26].
Results
Based on experimental data and applications in commercial olive groves, the proper management of boron can bring the following results:
Increased production:
Ben Rouina et al. (2013) recorded a 15-30% increase in production after correction of severe boron trophic deficiencies in experimental olive groves in Tunisia [27].
Improving quality characteristics:
Research by Rodrigues et al. (2011) showed that adequate boron supply increased the oil content by 5-8% and increased the concentration of phenolic compounds in olive oil, enhancing its organoleptic and nutritional characteristics [28].
Strengthening resilience:
Matichenkov et al. (2008) documented that adequate boron nutrition increased the drought tolerance of olive trees by 20-25% by improving stomatal function and water management [29].
Epilogue
As climate change brings new challenges to olive cultivation, optimizing boron nutrition is emerging as a very critical factor to ensure the sustainability and productivity of olive groves (Weisany et al., 2019) [30].
Bibliographical references
[1] Perica, S., Brown, P.H., Connell, J.H., Nyomora, A.M.S., Dordas, C., & Hu, H. (2001). Foliar boron application improves flower fertility and fruit set of olive. HortScience, 36(4), 714-716.
[2] Delgado, A., Benlloch, M., & Fernández-Escobar, R. (1994). Mobilization of boron in olive trees during flowering and fruit development. HortScience, 29(6), 616-618.
[3] Fernández-Escobar, R., Ortiz-Urquiza, A., Prado, M., & Rapoport, H.F. (2008). Nitrogen status influence on olive tree flower quality and ovule longevity. Environmental and Experimental Botany, 64(2), 113-119.
[4] Brown, P.H., Bellaloui, N., Wimmer, M.A., Bassil, E.S., Ruiz, J., Hu, H., Pfeffer, H., Dannel, F., & Römheld, V. (2002). Boron in plant biology. Plant Biology, 4(2), 205-223.
[5] Hu, H., & Brown, P.H. (1997). Absorption of boron by plant roots. Plant and Soil, 193(1-2), 49-58.
[6] Retalis, A., & Vachamidis, P. (2007).Effect of boron application on the growth and productivity of olive trees. Proceedings of the 12th Panhellenic Soil Science Conference, Thessaloniki, Greece, 142-148.
[7] Vossen, P.M., Berenguer, M.J., & Grattan, S.R. (2005). Olive Oil Production Manual. University of California, Agriculture and Natural Resources, Publication 3353.
[8] Chatzissavvidis, C., & Therios, I. (2010). Response of four olive (Olea europaea L.) cultivars to six B concentrations: Growth performance, nutrient status and gas exchange parameters. Scientia Horticulturae, 127(1), 29-38.
[9] Therios, I.N., & Dimassi, K.N. (2004). Nutrient deficiency symptoms in olive tree cultivars. In: Olivae, International Olive Oil Council, 101, 33-41.
[10] Tsadilas, C.D., & Chartzoulakis, K.S. (1999). Boron deficiency in olive trees in Greece in relation to soil boron concentration. Acta Horticulturae, 474, 341-344.
[11] Gupta, U.C., Jame, Y.W., Campbell, C.A., Leyshon, A.J., & Nicholaichuk, W. (1985). Boron toxicity and deficiency: a review. Canadian Journal of Soil Science, 65(3), 381-409.
[12] Goldberg, S., Lesch, S.M., & Suarez, D.L. (2000). Predicting boron adsorption by soils using soil chemical parameters in the constant capacitance model. Soil Science Society of America Journal, 64(4), 1356-1363.
[13] Nable, R.O., Bañuelos, G.S., & Paull, J.G. (1997). Boron toxicity. Plant and Soil, 193(1-2), 181-198.
[14] Peryea, F.J., Bingham, F.T., & Rhoades, J.D. (2003). Mechanisms of boron mobility in soils. In Gupta, U.C. (Ed.), Boron and its role in crop production (pp. 87-102). CRC Press.
[15] Dell, B., Huang, L., & Bell, R.W. (1997). Boron in plant reproduction. In: Boron in Soils and Plants (pp. 103-117). Springer.
[16] Fernández-Escobar, R., Sánchez-Zamora, M.A., García-Novelo, J.M., & Molina-Soria, C. (2009). Nutrient removal from olive trees by fruit yield and pruning. HortScience, 44(2), 478-480.
[17] Tsadilas, C.D., Dimoyiannis, D., & Samaras, V. (2005). Methods of boron assessment in relation to soil properties and olive plant performance. Communications in Soil Science and Plant Analysis, 36(1-3), 141-153.
[18] Soyergin, S., Moltay, I., Genç, Ç., Fidan, A.E., & Sütçü, A.R. (2010). Nutrient status of olives grown in the Marmara region. Acta Horticulturae, 868, 295-298.
[19] Fernández-Escobar, R., Morreno, R., & García-Creus, M. (2018). Seasonal changes of mineral nutrients in olive leaves during the alternate-bearing cycle. Scientia Horticulturae, 119(2), 210-213.
[20] Wojcik, P., & Wojcik, M. (2003). Effects of boron fertilization on ‘Conference’ pear tree vigor, nutrition, and fruit yield and storability. Plant and Soil, 256(2), 413-421.
[21] Camacho-Cristóbal, J.J., Anzellotti, D., & González-Fontes, A. (2002). Changes in phenolic metabolism of tobacco plants during short-term boron deficiency. Plant Physiology and Biochemistry, 40(12), 997-1002
