Microscopic attributes of Olea europaea L. leaves and bioclimatology: the case of Galicia, western Iberian Peninsula

Introduction The olive tree ( Olea europaea L.) is widely recognized for its capacity to persist under conditions of limited water availability, a feature often associated with leaf anatomical and histological traits. Methods In the present study, microscopic differences were comparatively analyzed in fully expanded leaves of five olive varieties sampled in Galicia (Brétema, Carapucho, Cobrancosa, Hedreira, and Mansa Gallega) and cultivated under controlled greenhouse conditions. Using light microscopy and scanning electron microscopy, we quantified adaxial cuticle thickness, palisade chlorophyll parenchyma thickness, leaf blade thickness, stomatal density, trichome density, and trichome morphometric traits. Additional qualitative traits were also assessed, including the presence of raphides and trichosclereids, the size of intercellular spaces in the aerenchymatous parenchyma, starch occurrence, the degree of curvature of the distal leaf portion, and the thickness of the vascular bundle sheath in the midrib. Results Significant intervarietal differences were detected, allowing the identification of two distinct clusters. Group I (Brétema and Carapucho) was characterized by greater cuticle and leaf blade thickness, higher trichome density, a thinner vascular bundle sheath, and smaller intercellular spaces. Group II (Cobrancosa, Hedreira, and Mansa Gallega) showed lower cuticle and leaf blade thickness, lower trichome density, a thicker vascular bundle sheath, and larger intercellular spaces. Discussion These results indicate contrasting histological configurations that may reflect different patterns of environmental adjustment among varieties. The observed traits suggest that Group I displays a more xeromorphic anatomical organization, whereas Group II shows features consistent with comparatively less structurally buffered leaf environments. Conclusions Based on these findings and following the global bioclimatic classification system of Rivas-Martínez, the most suitable bioclimatic areas in northwestern Iberia for cultivation of the studied olive varieties are proposed. This study highlights the histological variability of olive leaves and provides useful information for varietal selection in relation to different bioclimatic contexts, including future climate change scenarios.