Effects of long-term olive mill wastewater spreading on the physiological and biochemical responses of adult Chemlali olive trees (Olea europaea L.)

•Physiological and biochemical olive responses depend on OMW applied dose.•Net photosynthesis of olive trees was increased up to 100 m3 ha−1 year−1 OMW spread rate.•OMW spreading causes disturbance in olive tree performance when 200 m3 of OMW was applied.

Olive mill wastewater (OMW), a by-product from the olive oil extraction process, is annually produced in huge amounts in olive-growing areas and represents a significant environmental problem in Mediterranean areas. Whereas its high organic load and polyphenols concentrations are associated with troublesome environmental effects, its rich mineral and organic matter contents represent valuable nutrients. This study aimed to investigate adult ‘Chemlali’ olive trees subjected to OMW application at three doses (50, 100, and 200 m3 ha−1 year−1) after eight successive years in an olive field. The main focus of the study was on gas exchange characteristics, photosynthetic pigments, soluble sugars, total phenolic compounds, antioxidant activity, leaf mineral nutrient and yield of the trees. The findings showed significant raises of net photosynthesis (Pn) and stomatal conductance (Gs) in OMW treated olive trees by 50 and 100 m3 ha−1. The lowest Pn and Gs rates were recorded in treatment with the highest OMW dose (200 m3 ha−1). In addition, olive trees amended with 200 m3 ha−1 exhibited a significant decrease of both chlorophyll (a + b) and carotenoid contents, while olive plants which received 50 and 100 m3 ha−1 of OMW showed higher photosynthetic pigments contents compared to the control olive trees. However, there was no significant difference in carotenoid content. Soluble sugars, total phenol contents and antioxidant activity increment were noticed in leaf plants amended with a 200 m3 ha−1 dose. The OMW spreading rates at 50 and 100 m3 ha−1 significantly increased the mineral contents of leaves (N, P, K, Ca and Mg). Nevertheless, the highest OMW application was associated with a significant reduction of olive leaf nutrient concentrations. Olive yield showed a continuous improvement of up to 100 m3 ha−1 year−1 spread rate. The present study confirms that olive trees responses were dependent on OMW dose applied. Consequently, key physiological parameters of adult olive trees seemed to benefit from spreading OMW at moderate doses (50 and 100 m3 ha−1 year−1) while the application of a higher dose (200 m3 ha−1 year−1) seemed to disrupt them.