Proportion of oleic acid in olive oil as influenced by the dimensions of the daily temperature oscillation

Abstract

Olive fruit dry weight, oil concentration and the proportions of individual fatty acids in the oil are influenced by environmental variables, such as ambient temperatures, between flowering and harvest. An increase in mean daily temperature above 25 °C has been shown to have a negative effect on fruit dry weight, and to produce a linear decrease both in fruit oil concentration and oleic acid proportion in the oil over the range of 16–32 °C. Under natural conditions or in experiments in which mean daily temperatures are manipulated following the natural daily oscillation in temperature, mean daily maximum and minimum temperatures covary with mean daily temperature. However, variations in temperature associated with altitude, location and climate change can affect maximum and minimum temperatures differently and modify thermal amplitude. The objectives of the present study were to assess associations between changes in: i) yield variables (fruit dry weight and oil concentration) and ii) the proportions of major fatty acids in the oil, with the different dimensions of the daily temperature oscillation (mean daily minimum and maximum temperatures, mean daily thermal amplitude) experienced by the fruit during its growth from the pit-hardening stage to maturity. Five branch-level temperature treatments were applied: a control (T0) that followed the daily dynamics of ambient temperature, two levels of daytime (8–20 h) heating that increased temperature 5 and 10 °C relative to T0 during the day, and two levels of nighttime (20–8 h) heating to 5 and 10 °C more than T0. Treatments were applied for 76 days during the oil accumulation phase using transparent chambers with individualized temperature control to enclose fruiting branches of cultivar Arauco trees. The treatments successfully broke the natural covariance between the different dimensions of daily temperature variation, and achieved a broad range in mean daily temperature (∼6 °C) which covered the natural range of this variable for the region. Fruit dry weight showed a tendency to decrease with increasing mean temperature, while the proportion of oil in the fruit exhibited a significant relationship (R2 = 0.70) with mean daily thermal amplitude, and weaker −but significant- ones with mean daily maximum and minimum temperatures. The proportion of the main fatty acid in the oil, oleic acid, showed significant negative associations with mean daily minimum temperature (R2 = 0.45) and with mean daily temperature (R2 = 0.32), and a significant curvilinear relationship with mean daily thermal amplitude, but was not significantly associated with mean maximum temperature. Mean daily thermal amplitude in our experiment was determined mainly by mean daily minimum temperatures, a feature also found in an analysis of meteorological data for five sites and five years in the olive producing areas of La Rioja province, Argentina. Our results highlight the need to broaden studies on the temperature responses of olive fruit size, oil content and oleic acid content of the oil to include the effects of minimum temperature and thermal amplitude.