## 4 Discussion  
Effects of the Treatments on the Growth, Yield, and Fruit Characteristics of the Olive Tree and Quality Parameters of the Olive Oil  
Both deficit irrigation and the presence of _Ailanthus_ led to a reduction in oil content. Although several studies have reported increased oil content under deficit irrigation [38; 39], the observed decrease in this trial is likely the result of combined stress caused by both _Ailanthus_ presence and imposed water limitations. Moreover, water stress alone has been shown to reduce oil content compared to full irrigation conditions [18; 40]. Olive trees subjected to water deficits often exhibit reduced photosynthetic rates, primarily due to stomatal closure and limitations in mesophyll conductance [41; 42]. These physiological constraints lead to lower carbohydrate availability, directly impairing oil synthesis. Prolonged drought conditions can also trigger oxidative stress, which damages cellular structures and further reduces photosynthetic efficiency [41]. In addition to competing for water, _Ailanthus_ may negatively impact carbon assimilation through shading and nutrient competition, both of which can contribute to decreased oil content. In the present experiment, during the third year, _Ailanthus_ height reached, in most plots, almost half the height of the olive trees, generating in general a slight shading effect. [PERSON] et al. [43] found that shaded olive trees display lower photosynthetic rates and increased photoinhibition, especially under drought stress. Shading has also been shown to hinder oil accumulation, as oil biosynthesis between late summer and early winter relies heavily on photosynthetic activity [44]. A reduction in photosynthetically active radiation (PAR) limits carbohydrate reserves, which negatively affects oil accumulation in the fruit [44]. Nutrient imbalances may further exacerbate this effect. Adequate levels of calcium (Ca), potassium (K), and boron (B) have been positively correlated with higher oil content [45]. Potassium, in particular, is essential for stomatal regulation and enzyme activation during photosynthesis; its deficiency can impair photosynthetic efficiency and fruit development [46]. Likewise, nitrogen (N) and phosphorus (P) are critical for chlorophyll synthesis and energy transfer, and their limited availability can significantly constrain the plant's photosynthetic capacity [47]. Finally, beyond these direct competitive effects, the presence of _Ailanthus_ may also influence olive tree health and oil yield through its allebotathic properties [2; 5; 6]. In the present trial, deficit irrigation resulted in reduced fruit size, regardless of the presence of _Ailanthus_. This finding aligns with previous studies [18; 48], which reported greater fruit weight in trees under full irrigation. Similarly, [PERSON] et al. [49] observed that Manzanilla cultivar trees subjected to severe water stress produced the smallest fruits in terms of both weight and dimensions. Moreover, fruit growth parameters--such as diameter, length, and weight--have been shown to correlate directly and positively with plant water consumption [50]. Deficit irrigation also led to a lower fruit maturity index. Water deprivation has been shown to delay fruit ripening, resulting in a lower maturity index at harvest and altering the pace of fruit development, as observed in 'Arbequina' olives [51]. [PERSON] et al. [52] similarly reported changes in fruit characteristics associated with reduced irrigation. These findings suggest that limiting water availability can slow the maturation process, ultimately leading to a lower maturity index. Additionally, a significantly lower chroma value was recorded in fruits from fully irrigated trees, consistent with the findings of [PERSON] et al. [48], which may indicate an advancement in fruit maturation. Deficit irrigation significantly reduced the free acidity of olive oil compared to full irrigation, while it had no effect on peroxide value or ultraviolet absorbance indices, in agreement with [PERSON] et al. [38].