Oil yield

Mariela Torres: Estación Experimental Agropecuaria San Juan, Instituto Nacional de Tecnología Agropecuaria (INTA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), San Juan, Argentina. Pierluigi Pierantozzi, Mariela Torres: Ingeniería Agronómica, Facultad de Ingeniería, Universidad Nacional de San Juan. Damián Maestri: Instituto Multidisciplinario de Biología Vegetal (IMBIV), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) - Universidad Nacional de Córdoba (UNC), Córdoba, Argentina.

One of the first questions asked by people who want to establish an olive grove is: how much olive oil can I get from my trees? There is no simple answer to this question as many factors interact and affect oil yield. Another common question is: how many olives are needed to produce one litre of olive oil? This depends on the fruit oil content of the particular olive variety -there are hundreds of them with different oil synthesis capacities. In some varieties, oil accumulation in the fruit is low (these are usually intended for table olive production). By contrast, other varieties accumulate high percentages of oil. Most olive varieties have intermediate oil contents (Figure 1).

Although the maximum amount of oil that an olive variety can produce is genetically determined, it may be influenced by several factors. These can be grouped into five dimensions which also have sub-components: 1) Olive cultivars; 2) Growing environments; 3) Climate and weather; 4) Agronomic management; 5) Orchard design (Figure 2).

Figure 1. Oil yield categories for different olive cultivars. Oil yield is estimated based on the fresh weight of the fruit.

Figure 2. Factors affecting olive oil yield.

To better understand the influence that the agro-ecological environment can have on the synthesis and accumulation of oil in the olive fruit, it is important to consider that fruit oil accumulation starts in early fruit growth stages and increases during the summer to continue until the fruit ripens towards the end of autumn, when it levels off (Dag et al., 2011, 2014; Bellincontro et al., 2013; Bodoira et al., 2015). It is generally accepted that the process of oil accumulation during the development of the olive fruit follows a sigmoid-shaped curve with three more or less distinct phases (Frias et al., 1991). The first phase of slow accumulation characterises the newly formed fruit and takes place until the hardening of the endocarp. In the second phase of rapid accumulation, the quantity of oil increases linearly. The duration of this phase is variable as it can be significantly affected by environmental conditions. Towards the end of this phase, the first changes in the colour of the epidermis occur, marking the beginning of the ripening stage. The third phase coincides with a progressive slowing down and eventual the cessation of lipid synthesis. This stationary phase lasts approximately four weeks and precedes the onset of olive over-ripening. Normally, at 28/31 weeks after full bloom the oil is fully formed and a longer waiting time at harvest servers mainly to decrease the water content present in the fruit, thus increasing the extractability of the lipid fraction (Beltrán et al., 2017; Bodoira et al., 2015). Under the environmental conditions in the Mediterranean basin, most oil accumulation coincides with the late summer and autumm months, when temperatures are decreasing from maximum summer values.

Increasing evidence suggests that stress conditions during lipid biosynthesis, mainly water stress and high air temperatures, lead to a reduction in oil synthesis capacity. 

High water stress during the oil accumulation phase decreases fruit oil content (Lavee, 1991). However, moderate water reductions do not always decrease the amount of oil, and, in addition to  enabling a more efficient use of water resources (maximising fruit and/or oil yield per unit of water used), they improve quality and key aspects of crop management (Patumi et al., 2002; Pierantozzi et al., 2020). Very little information is available on differences in the effect of water stress on oil accumulation by cultivar.

The thermal regime of the growing environment can also have a significant influence on the oil synthesis capacity of olive fruits. Perhaps the earliest observations on this phenomenon were made in Israel, where fruits from trees of cv. Manzanillo accumulated 4 % (fresh weight basis) less oil when they were grown in a warm interior valley compared to those of a cooler coastal plain (Lavee et al., 2012). 

A dataset of 113 olive cultivars evaluated in nine environments in five countries (Italy, Spain, Morocco, Lebanon and Argentina) was used to calculate Pearson’s correlation coefficients between ten-day temperature series and average values of oil content (Mousavi et al., 2019). The correlation analyses between the environmental temperature and the fruit oil content showed that the highest values of maximum average temperature during the colder months of the year were positively correlated with an increase in fruit oil content, but the correlation becomes negative if temperatures rose substantially during the summer. In addition, high thermal amplitude during the summer months decreased the oil quantity on a dry weight basis.

Several studies carried out in olive orchards located in areas other than the Mediterranean region can serve as a basis for the choice of planting sites for olive trees intended for oil production. In some regions of the Southern Hemisphere, particularly in Argentina, olive growth takes place under thermal regimes usually warmer than those observed in the Mediterranean region. This affects the phenology of the crop, so that the fruit development and the oil synthesis period in some Argentinian environments occur mostly under warm summer temperatures (Torres et al., 2017). This fact does not seem to affect the overall, basic pattern of oil accumulation in the fruit (Trentacoste et al., 2012; Bodoira et al., 2015), which occurs in a similar way to that stated above. In contrast, the amount of oil accumulated does seem to be greatly affected by high temperatures. Based on correlative studies under field conditions in warm regions of north-west Argentina, Rondanini et al. (2014) found that, combining data from several cultivars (Arbequina, Arauco, Barnea, Coratina, Frantoio and Manzanilla Fina), the final fruit oil content was negatively associated with the mean daily temperature (range of 23° - 27°C) averaged over the entire oil accumulation period, so that oil content dropped 3% (dry weight basis, DWB) for each ºC increase. The average oil contents from these environments (36.5 - 48.5%, DWB) were also lower than those found in cooler environments in central-western Argentina (45.5 - 57.4%; Trentacoste et al., 2012).

An important approach to directly evaluate the oil yield response to high temperature was through in manipulative experiments using open-top chambers with controlled temperature conditions. Significant changes in the fruit oil content were recorded after 4 months of heating or cooling of fruiting branches (cv. Arauco). In the full range of temperatures explored (16 – 32ºC), the oil content decreased linearly at 1.1% for each ºC of temperature increase. Likewise, in short-term experiments (each lasting one month), a temperature increase of 7°C above the control (ambient temperature) significantly decreased the oil content, particularly when the exposure to high temperature took place in the initial phase of oil accumulation (García-Inza et al., 2014).

A recent study conducted with the cultivars Arbequina and Coratina complements the findings mentioned above. Also, by using controlled experimental conditions, Miserere et al. (2023) report that a warm-up treatment (3°C above the ambient temperature) reduces the oil accumulation rate-but not the duration (i.e., days) of oil accumulation - compared to a control treatment (maintained near the ambient temperature). The result was a significant decrease in the final oil contents in both cultivars. 

Although studies carried out under controlled temperature conditions are still scarce and involve a small number of cultivars, the results are consistent with those observed in field experimental trials. Overall, the studies suggest the thermal regime as a major factor affecting the oil content in olive fruits. The most important consequence of this would be the lower oil yields generally observed in high-temperature environments. 

Considering the commercial importance of a high oil content in the fruit, it is of great interest to characterise not only the genetic variability that may be present in this trait, but also to identify those genotypes with stable oil yield responses in diverse environments. Understanding the influence of the environmental growth temperature on olive oil synthesis and accumulation is also relevant in the current global warming scenario. All of these aspects could contribute to the selection of suitable growing environments and genotypes that provide high oil yields. 

It is important to note, however, that the environmental growth temperature does not affect oil synthesis equally in all olive cultivars. For instance, a study using several cultivars grown at high and moderate ambient temperatures reports non-significant effects of high temperatures on the oil contents in Barnea, Coratina and Picholine. In contrast, at this latter condition, the dry fruit oil content decreased 15% and 8% in Koroneiki and Souri, respectively (Nissim et al., 2020). 

In addition to temperature, light can also affect oil synthesis. In general, it can be observed that olives grown under well-lit conditions are heavier, have a higher percentage of oil and a lower water content compared to those grown at low light intensity (Proietti et al., 1994, 2011; Cherbiy-Hoffmann et al., 2013).This simple conclusion has numerous implications at the practical level, as planting models and pruning  can greatly influence the amount of light received by the trees (Reale et al., 2019). This could be of importance especially in super-intensive planting models where row orientation could influence the quantity, availability and quality of light received by plants. However, in the experimental trials and latitudes in which these trials have been conducted, hedge planting orientation does not seem to affect the amount of oil produced (Gómez del Campo et al., 2022). These latter observations, however, should not be generalised, given that the response could change in environments with more limited lighting. 

On the other hand, negative correlations have been observed between yield and pulp oil content. In years when yields are high, trees tend to produce fruits with a smaller size and lower oil content than those obtained in years of low production (Lavee and Wonder, 1991). So far, there is no certain explanation for these observations, and the reasons given to explain such differences are often controversial.

Figure 3. Main phenological stages of olive cultivation in different olive-producing regions in the world. Reprinted from “Olive cultivation in the southern hemisphere: flowering, water requirements and oil quality responses to new crop environments”, by Torres et al. (2017), Front. Plant Sci. 8, 1830.

Gucci et al. (2007) hypothesised that the decrease in fruit oil concentration in high fruit load trees compared to their low fruit load counterparts is associated with reduced availability of plant resources affecting oil synthesis rather than fruit growth. On the other hand, Trentacoste et al. (2010) have shown that fruit load affects fruit size primarily through changes in fruit growth rate (i.e. low source–sink ratios such as < 2m3 1000 fruit−1), thus decreasing both oil weight per fruit and fresh fruit weight, which results in a conserved fruit oil concentration. The colouring of fruits was dramatically affected by fruit load, but the duration of fruit growth and dynamics of oil accumulation in the fruit were unaffected by fruit load. 

In relation to nutrient availability, a recent study suggests an inverse relationship between oil biosynthesis and fruit nitrogen level (Erel et al., 2023). Fruit nitrogen availability increased in response to nitrogen fertilization level, and it was inversely related to fruit load. The negative correlation between fruit nitrogen and oil content suggests that the protein/oil trade-off paradigm cannot explain the noticeable decrease in oil biosynthesis in olives, suggesting that additional mechanisms could be involved in nitrogen-induced inhibition of oil biosynthesis. It is worth mentioning that such inhibition is not related to the soluble carbohydrate levels in the fruit, which were comparable regardless of nitrogen level (Erel et al., 2023). 

Overall, this background information highlights the need for further studies to better understand how much phenotypic variability can be mainly attributed to genetic control, in order to enhance the prediction of crop performance across diverse environments or design better management strategies. Furthermore, it emerges that studies on olive germplasm and comparative trials under different environments are essential to predict the behaviour of specific varieties and/or new genotypes in view of widening the area of olive cultivation, particularly regarding new environments not previously experienced by the olive crop, and in light of the current global climate change scenario. 

References
   1. Bellincontro A., Caruso G., Mencarelli F., Gucci R. 2013. Oil accumulation in intact olive fruits measured by near infrared spectroscopy–acousto-optically tunable filter. J. Sci. Food Agric. 93, 1259–1265. doi: 10.1002/jsfa.5899.
   2. Beltrán, G., Uceda, M., Hermoso, M, Frías, L. 2017. “Maduracion” in El Cultivo del olivo. Eds. Barranco D., Fernández-Escobar R., Rallo L. (Junta de Andalucía / Mundi Prensa / RIRDC / AOA) 187-209.
   3. Bodoira, R., Torres, M., Pierantozzi, P., Taticchi, A., Servili, M., and Maestri, D. 2015. Oil biogenesis and antioxidant compounds from Arauco olive (Olea europaea L.) cultivar during fruit development and ripening. Eur. J. Lip. Sci. Technol. 117, 377–388. doi: 10.1002/ejlt.201400234.
   4. Cherbiy-Hoffmann, S.U., Hallb, A.J., Rousseaux, M.C. 2013. Fruit, yield, and vegetative growth responses to photosynthetically active radiation during oil synthesis in olive trees. Scientia Horticulturae 150: 110-116.
   5. Dag, A., Kerem, Z., Yogev, N., Zipori, I., Lavee, S., Ben-David, E. 2011. Influence of time of harvest and maturity index on olive oil yield and quality. Sci. Hortic. 127: 358-366.
   6. Erel, R., Yermiyahu, U., Yasuor, H., Ben-Gal, A., Zipori, I., Dag, A. 2023. Elevated fruit nitrogen impairs oil biosynthesis in olive (Olea europaea L.). Front Plant Sci. 14:1180391. doi: 10.3389/fpls.2023.1180391. 
   7. Frías, L., García- Ortiz, A., Hermoso, M., Jiménez, A., Llavera, M.P., Morales, J., Ruano, T.; Uceda, M., 1991. Analistas de laboratorio de almazaras. Series Apuntes 6/91. Consejería de Agricultura, Ganadería, Pesca y Desarrollo Sostenible. Junta de Andalucía.
   8. García-Inza, G.P., Castro, D.N., Hall, A.J., Rousseaux, M.C. 2014. Responses to temperature of fruit dry weight, oil concentration, and fatty acid composition in olive (Olea europaea L. var. ´Arauco´). Eur. J. Agron. 54: 107-115.
   9. Gómez-del-Campo, M., Trentacoste, E.R. and Connor, D.J. 2022. Long-term effects of row orientation on oil yield and oil yield components of hedgerow olive orchards cv. Arbequina. Scientia Horticulturae 294, 110770.
   10. Gucci, R., Lodolini, E., Rapoport, H.F. 2007. Productivity of olive trees with different water status and crop load. J. Hortic. Sci. Biotech. 82: 648-656.
   11. Lavee, S., Haskal, A., and Avidan, B. 2012. The effect of planting distances and tree shape on yield and harvest efficiency of cv. Manzanillo table olives. Sci. Hortic. 142, 166–173. doi: 10.1016/j.scienta.2012.05.010.
   12. Lavee, S., Wodner, M. 1991. Factors affecting the nature of oil accumulation in fruit of olive (Olea europaea L.) cultivars. J. Hortic. Sci. 66: 583-591
   13. Miserere, A., Searles, P.S., Rousseaux, M.C. 2023. Influence of experimental warming on the rate and duration of fruit growth and oil accumulation in young olive trees (cvs. Arbequina, Coratina). Plants (Basel). 12(10):1942. doi: 10.3390/plants12101942.
   14. Mousavi, S., de la Rosa, R., Moukhli, A., el Riachy, M., Mariotti, R., Torres, M., Pierantozzi, P., Stanzione, V., Mastio, V., Zaher, H., et al. 2019. Plasticity of fruit and oil traits in olive among different environments. Sci Rep 9, 16968.
   15. Nissim, Y., Shloberg, M., Biton, I., Many, Y., Doron-Faigenboim, A., Zemach, H., Hovav, R., Kerem, Z., Avidan, B., Ben-Ari, G. 2020. High temperature environment reduces olive oil yield and quality. PLoS One 15, e0231956.
   16. Patumi, M., d’Andria, R., Marsilio, V., Fontanazza, G., Morelli, G., Lanza, B. 2002. Olive and oil quality after intensive monocone olive growing (Olea europaea L. cv. Kalamata) in different irrigation regimes. Food Chem. 77: 27-34.
   17. Pierantozzi, P., Torres, M., Tivani Keaik, M., Contreras, C., Gentili, L., Parera, C., Maestri, D. 2020. Spring deficit irrigation in olive (cv. Genovesa) growing under arid continental climate: effects on vegetative growth and productive parameters. Agric. Water Manag. 238, 106212.
   18. Proietti, P., Nasini, L., Ilarioni, L., Balduccini, A.M. 2011. Photosynthesis and vegetative-productive activities of the olive cultivars Arbequina, Leccino and Maurino in a very high density olive grove in Central Italy. Acta Hort. 924:111–116.
   19. Proietti, P., Tombesi, A., Boco, M. 1994. Influence of leaf shading and defoliation on oil synthesis and growth of olive fruit. – Acta Horticult. 356: 272-277.
   20. Reale, L., Nasini, L., Cerri, M., Regni, L., Ferranti, F., Proietti, P. (2019) The Influence of Light on Olive (Olea europaea L.) Fruit Development Is Cultivar Dependent. Front. Plant Sci. 10:385. doi: 10.3389/fpls.2019.00385.
   21. Rondanini, D.P., Castro, D.N., Searles, P.S., Rousseaux, M.C. 2014. Contrasting patterns of fatty acid composition and oil accumulation during fruit growth in several olive varieties and locations in a non-Mediterranean region. Eur. J. Agron. 52: 237-246.
   22. Torres, M., Pierantozzi, P., Searles, P., Rousseaux, M. C., García-Inza, G., Miserere, A., Bodoira, R., Contreras, C., Maestri, D. 2017. Olive Cultivation in the Southern Hemisphere: Flowering, Water Requirements and Oil Quality Responses to New Crop Environments. Front Plant Sci, 8.
   23. Trentacoste, E.R., Puertas, C.M., Sadras, V.O. 2010. Effect of fruit load on oil yield components and dynamics of fruit growth and oil accumulation in olive (Olea europaea L.). Eur. J. Agron. 32: 249-254. 
   24. Trentacoste, E.R., Puertas, C.M., Sadras, V.O. 2012. Modelling the intraspecific variation in the dynamics of fruit growth, oil and water concentration in olive (Olea europaea L.) Eur. J. Agron. 38: 83-93.