Linking Leaf Structure to Function in Olive (Olea europaea L.): Implications for Cultivar Selection and Resource-Use Strategies

The olive tree (Olea europaea L.) holds a prominent place in Mediterranean cultural and agricultural landscapes, including Italy, which hosts a vast germplasm of almost 600 cultivars. Characterising this biodiversity is important for conservation, sustainable use, and breeding programmes. This study provides a characterisation of leaf morpho-anatomical and physiological traits in 24 Mediterranean olive cultivars grown under uniform agronomic conditions, allowing the assessment of genotype-driven variability. The research investigates leaf morpho-anatomical characteristics, from macroscopic traits (leaf size, shape, and colour) to anatomical traits of tissues, trichomes (including the trichome area index, TAI), and stomata, together with physiological parameters (gas exchange and chlorophyll a fluorescence). Significant differences were observed among cultivars in almost all measured traits. The main results include substantial variation in leaf area (339–872 mm2), lamina thickness (331–489 µm), and ratio of palisade to spongy tissue. Trichome density had a wide range of variation (a threefold increase from the lowest to the highest value, 71–206 mm−2), and TAI values revealed that some cultivars possess up to three overlapping trichome layers. This multifunctional phenotypic approach proves to be a useful tool for discriminating cultivars and understanding their functional strategies. Multivariate analysis (PCA) showed patterns of association among structural and physiological traits, highlighting different trait combinations across the germplasm. These results indicate a linkage between leaf structure and function and help to interpret differences in gas exchange and water-use efficiency among cultivars. Overall, the results highlight the value of germplasm collections and provide useful insights for cultivar selection and the sustainable management of olive germplasms.