Effect of regulated and sustained irrigation on oil concentration and minor compounds of olive oil

Deficit irrigation strategies are one of the chief alternatives to cope with water shortages in the actual context of climate change. Regulated (RDI) and sustained deficit irrigation (SDI) are the main deficit irrigation strategies that have been adopted in irrigated olive orchards. The success of the RDI depends on the exact period to impose the water deficit. The objective of this work was to evaluate and compare the effect of different strategies of RDI and SDI on fruit growth and oil accumulation on ‘Cobrançosa’, during the 2019 and 2020 seasons, in an olive orchard located at the northeast of Portugal. Two sustained deficit irrigation were tested, with 60 and 30% (SDI₃₀) of control (FI) plus two RDI, one irrigated as the control (FI) except in the pit hardening (PI) period (reduced to 10% of FI), and the other the irrigation cut-off in this phenological stage, after that was irrigated equally to SDI₆₀. A good relationship was observed between oil concentration (r²=0.89; p<0.01) and the total water stress integral, which was evaluated by midday shoot water potential. Oil concentration in the fruits varied between 32.4 and 32.9% (SDI₃₀) to 43.1-47.9%(FI). Oil concentration and yield of RDI₁₀₀ was not statistically different to the control. Moreover, oil content fruit^‑1 (mg oil/fruit dry weight) was affected by water stress except in RDI₁₀₀ treatment. Olive oil obtained from the most stressed treatment were the richest in minor compounds, like vitamin E and polyphenols derivates of secoiridoids (hydrolyzed hidroxitirosol and tyrosol). Results from 2020 harvest indicated that all the oil from olives subjected to deficit irrigation treatments and from the control met the health claim on polyphenols. The RDI strategies had a positive effect over the two SDI strategies, especially the RDI₁₀₀, as it increased oil yield nearly to double in relation of SDI₆₀ and more than double to the SDI₃₀. Also, The SDI₆₀ treatment had a very positive effect over SDI deficit irrigation strategies. In conclusion, RDI₁₀₀ seems to be a good DI to implement on ‘Cobrançosa’ in this region. For a better performance of the RDI₆₀, it seems that to supress irrigation on pit hardening period should be replaced by reducing the volume of applied water to 10-20% of FI, namely in soils with low water storage capacity.