## 4 Discussion

Table 2: The number of leaves per plant (NL), average leaf area (ALA), total leaf area (TLA), and leaf mass per area (LMA) were measured or calculated in non-inoculated lettuces (C) and in lettuces where the SynCom was applied to the roots (R), to the leaves (L), or both roots and leaves (RL). Mean values and standard errors (s.e.) are reported. Distinct uppercase letters denote statistically significant variations (_p_ < 0.05) among treatments for individual parameters, assessed independently for each parameter (n = 5). and waxes overlying the epidermal cells, contributes to the nutrient-limited conditions characteristic of the phyllosphere by restricting the transport of water and polar molecules, thereby limiting nutrient availability for microbial colonization [60]. These issues are particularly pronounced in Mediterranean ecosystems, where factors like drought, high salinity, and nutrient scarcity select for stress-tolerant microbial communities [23]. Moreover, sclerophyllous Mediterranean species are typically characterized by a thick cuticle, which has been proven to be negatively correlated with a bacterial population size of the phyllosphere [60]. We focused on the leaf microbiome of _P. lentiiscus_, a representative Mediterranean evergreen sclerophyll species with an abundant culturable epiphyytic bacterial population [34], to evaluate its potential to serve as a reservoir for PGPR strains suitable for foliar application. Leaf samples were collected in different vegetation types of Northern Sardinia in the late summer after a prolonged dry period, which could have potentially enriched the leaf in strains tolerant to high temperature, water stress, and UV radiation. Independently from the vegetation type, all strains isolated from the phyllosphere of _P. lentiiscus_ were able to grow in a medium amended with 5% (_w/v_) NaCl, and most of them tolerated a NaCl concentration of up to 10% (_w/v_) (Table 1). Two strains from the garrigue and one from the pure plantation developed larger colonies when grown in the 7.5% NaCl (_w/v_) amended medium, exhibiting a halophilic behavior. The highest salt tolerance was observed among isolates (100%) from the pure plantation (PP) and the heavy metal-contaminated site (M) (Table 1). This could be related to the lower canopy stratification of these sites compared to the garrigue (G) and mixed plantation (MP), where _P. pinea_ L. (G), _J. phohenicea_ L. (G and MP), and _R. officinalis_ (G and MP) were present in the dominant layer. This vertical stratification decreased light penetration and attenuated maritime winds on _P. lentiiscus_ in the understory, creating a microclimate with reduced environmental stressors (UV radiation, thermal stress, and salt deposition) compared to the PP and M sites. These conditions may have affected the differential distribution of halotolerant strains in the _P. lentiiscus_ L. phyllosphere, as shown in Table 1.  
Analysis of the phosphate-solubilizing and N-fixing activity of the different isolates also revealed an effect of the vegetation type on the enrichment of PGPR (Table 1). Interestingly, the highest percentage of strains with PGPR traits was observed in the mixed plantation, a site in which it has been shown that there is a heterogeneous distribution of the soil N contents, with lower values under _P. lentiiscus_ canopies compared to the other co-occurrent species (_J. phohenicea_ L. and _R. officinalis_) [61]. This result agrees with [PERSON] et al. [35], who studied the effect of encroachment by six Mediterranean plant species, namely _P. lentiiscus_ L., _J. phohenicea_ L., _Myrtus communis_ L., _R. officinalis, Olea europaea_ L., and _Euphoria dendroides_ L., on soil microbial communities. Soils associated with _P. lentiiscus_ L. had the lowest N content and highest abundance of free-living N-fixing bacteria.