NPK nano-fertilizers enhance growth, oil quality, and yield regularity in Picual olive trees

This study aimed to assess the impact of green-synthesized NPK nano-fertilizers (NPKNF) on the growth, productivity, and quality of the 'Picual' olive cultivar under Egyptian conditions. Foliar sprays of NPKNF at 0 (control), 2000, 3000, and 4000 mg L^-1 were applied during the "on-year" season. Evaluations covered vegetative growth, leaf and fruit mineral content, fruit traits, yield, oil and moisture content, and various olive oil quality indicators. In the following "off-year" season, flowering, fruit set, yield, and alternate bearing severity were also examined. Results indicated a significant enhancement in vegetative growth, fruit traits, yield, and oil properties during the "on-year" season in response to increasing NPKNF concentrations. The highest concentration (4000 mg L^-1) demonstrated the most pronounced improvements across all evaluated parameters compared to the untreated control. Treated trees produced larger fruits with an increased pulp-to-pit ratio and a higher oil content. Oil extracted from NPKNF-treated fruits exhibited a measurable reduction in total phenolic compounds (289.28, 298.59, and 277.11 mg/kg with 2000, 3000, and 4000 mg L^-1, respectively) compared to the untreated control (350.51 mg/kg). These compounds act as natural antioxidants and contribute to the nutritional value and organoleptic properties of olive oil. Despite this reduction, all olive oil samples were classified as extra virgin, with sensory analysis indicating a moderate fruitiness level across treatments. Furthermore, an improvement in olive flowering, fruiting parameters, and yield during the "off-year" was observed, along with a reduction in the severity of alternate bearing in response to NPKNF treatments. One of the most significant outcomes of the study, consistent with its objective, is the significant increase in the number of new shoots per shoot during the "on-year," reaching 5.09 with NPKNF 4000 compared to 1.03 in the control. This increase contributes to the next season's yield ("off-year") by reducing the impact of alternate bearing, as evidenced by the significant difference in severity values, with 85.40 for NPKNF 4000 and 92.66 for the control. These findings emphasize the potential role of NPKNF treatments in enhancing olive fruit quality and oil properties, while also contributing to the mitigation of alternate bearing severity.