Changes in key photosynthetic parameters of olive trees following soil tillage and wastewater irrigation, modified olive oil quality

- Photosynthetic key parameters increased under WW irrigation and soil tillage.
- Under rainfed conditions and No-tilled soil, stress metabolites increased.
- Oils produced from TNTS had the best quality and nutritional value.
- WW irrigation affected the antioxidant profile and fatty acid composition of olive oil.

The use of wastewater (WW) provides a reliable source of water and nutrients for crop production. Soil tillage is one of the key soil management practices in agricultural land use. In the present study, we studied the effects of these two agricultural practices on key physiological parameters of olive trees and on olive oil quality. The experiment was carried out on mature olive trees (Olea europaea L. cv. Chemlali). Two tillage practices treatments (Trees grown in Tilled soil: TTS; and Trees grown in No-tilled soil: TNTS) were combined with two irrigation treatments (Trees irrigated with WW: IT; and Trees grown under rainfed condition: TRC) during two successive years (2013 and 2014). Statistical analyses demonstrated that the factor “soil tillage” was more efficient in improving photosynthetic parameters of olive trees than WW irrigation. The combination of soil tillage with WW irrigation improved significantly leaf chlorophyll fluorescence parameters, stomatal conductance, transpiration and chlorophyll content, which closely associated with an increase of photosynthetic rate, and a decrease of stress metabolites (leaf phenolic compounds and carbohydrates). On the other hand, a decrease of key photosynthetic parameters and an accumulation of stress metabolites in leaves were observed when combining rainfed condition and No-tilled soil. WW irrigation affected negatively the oxidative stability and nutritional value of olive oil, especially by decreasing the contents of antioxidant compounds (total phenols and pigments) and the levels of MUFA, C18:1, C18:3 and elevating SFA and PUFA contents. However, soil tillage combined with TRC improved oil stability to oxidation and its nutritional value by increasing the contents of antioxidant compounds and the levels of MUFA and C18:1, as well as C18:1/C18:2 and MUFA/PUFA ratios.The results of this study prove that, under water scarcity conditions, wastewater irrigation in oleiculture should be combined with soil tillage and recommended only in the 'Off'-year to support vegetative growth and improve physiological performance of olive trees. In the 'On'-year, tilling the soil under rainfed conditions proved to be the most favorable cropping strategy for enhancing both physiological performance of olive trees and olive oil quality. These findings must be taken into consideration when developing strategies for oil quality enhancement, mainly in an exporting country like Tunisia.