Abstract
The olive tree (Olea europaea L. subsp. europaea var. europaea) is currently one of the most widely cultivated temperate fruit trees in the world. It is characterised by a large genetic diversity, as well as a large variation in phenotypic traits. Across the Mediterranean Basin, the nuclear genetic diversity of olive is organized into three main groups: ‘West’, ‘Central’, ‘East’, with an additional group (‘Mosaic’ or ‘admixed’) corresponding to varieties resulting from the hybridization of varieties belonging to at least two of these three groups. The first aim of the present study is to test whether the phenotypic traits of the cultivated varieties differ among these four genetic groups. To do so, nine leaf traits and five pit traits were measured on 35 varieties belonging to these four groups. The second aim of this study is to use this combination of traits to assess whether some trait covariations previously observed across and/or within species hold within olive, a sclerophyllous species characteristic of the Mediterranean Basin. We found that: (1) Trait values span a wide range of variation across varieties, from approximately 7-fold for pit volume, the most variable trait, to 1.3 fold for leaf dry matter content, the least variable one; (2) For most traits, there was no significant difference among the four genetic groups recognized across the Mediterranean Basin; (3) Leaf size was positively related to leaf mass per area, a trait describing the morpho-anatomy of the leaf; this is the consequence of leaf mass increasing more rapidly than leaf area, and agrees with the hypothesis that bigger leaves require to be stiffer to compensate for mechanical damage and gravity; (4) The two underlying components of leaf mass per area, leaf thickness and density, contributed equally to its determination; (5) Leaf and pit sizes were positively related. A structural equation modelling approach allowed us to show that this relationship was mediated through leaf mass per area, which is interpreted in the context of the recently proposed “Seed-Phytometer-Leaf” model.Perspectives to this work include: (a) further testing of the lack of phenotypic differences among genetic groups by increasing the number of varieties studied; (b) collecting data on fruits and twigs to test the underlying allometric model linking pit and leaf size, and (c) assessing how several leaf traits determined in this study - leaf area, leaf width, leaf mass per area - relate to gas exchange and plant water economy, so as to better understand how the different olive varieties cope with the contrasting climatic conditions from which they originate. This work would strengthen the functional bases for the selection of olive varieties.