Sap flow as a tool to enhance fruit quality: the case of olive oil

Olive oil is considered as one of the healthiest oils for human consumption due to its high amount of monounsaturated fats and also to the content of antioxidants, polyphenols and triterpenes. Among these last compounds, the oleanolic acid and maslinic acid are of great interest because they are shown to have anti-tumoral and anti-cancer activity. Interestingly, water stress has been observed to enhance these compounds in olive oil. Thus, an irrigation tool based on tree physiology to manage the water stress level would be useful for improving health benefits of olive oil. The general objective of our work was to develop an irrigation approach, based on sap flow-related measurements, to evaluate and control the water stress of olive trees in order to increase the above-mentioned compounds in olive. To achieve this objective we conducted an experiment having four irrigation treatments, where we applied 100, 75, 50 and 25% of the irrigation needs. Sap flow gauges were installed in 16 olive trees, four replicates for each irrigation treatment, to estimate sap flux density by the compensation heat flux method. Sap flux density was used to estimate stomatal conductance in a continuous manner, using a method developed previously for olive. This simulated stomatal conductance was linked to a biochemical model of CO2 assimilation to estimate daily photosynthesis. Moreover, we determined the carbon isotope ratio (delta C-13) in oil of the same monitored trees, as well as the oil compounds above-mentioned. Our results showed that those trees with a higher water supply had higher accumulated photosynthesis on a daily and seasonal basis. Oil delta C-13 showed the same trend, confirming the effect of stomatal closure on reduction of oil synthesis and the robustness of our methodological procedure to estimate gas exchange from sap flux density dynamics. Furthermore, the level of stomatal conductance achieved correlated inversely with the concentrations of oleanic acid and maslinic acid in the oil. Our results encourage the potential use of sap flow-related measurements to manage irrigation and control the desired level of water stress based on physiological criteria, which could be useful to improve the content of relevant compounds in olives.