Correspondence between genetic data and leaf morphological and anatomical traits in two Mediterranean Quercus and their hybrids along environmental gradients.

Oaks are known for their frequent hybridization, a trend that generates a wide phenotypic spectrum and produces taxonomic confusion within the genus. Different approaches are needed for hybrids characterization, such as a combination of leaf morphological and anatomical traits supported by molecular data. Here, we characterized some morphological and anatomical (veins and stomata) leaf traits and their changes across a temperature gradient of two closely related Mediterranean Quercus species (Q. faginea Lam. and Q. pyrenaica Willd.) and their hybrids, preliminarily identified from molecular markers. Quercus faginea tended to exhibit more drought-adapted traits (smaller leaf size, larger vein and stomatal density, but lower stomatal index) than Q. pyrenaica. Quercus faginea also tended to exhibit stronger responses to environmental changes between the climatic zones. Hybrids possessed intermediate stomatal and vein traits between the two parent species but they were similar to Q. faginea for most morphological traits. For a few leaf shape parameters, such as the length/maximum width ratio, hybrids tended to exhibit larger values than both parent species. Whether this transgressive character of the hybrids affects their fitness in the contact zones between the two parent species needs to be better investigated. For the three genetic groups there were consistent changes in morphological traits across the climatic gradient. By contrast, stomatal and vein traits did not show significant within-species changes across the gradient. Leaf size decreased with decreasing temperatures, mainly due to a strong reduction of maximum leaf width. It is known that a reduced leaf size leads to the reduction of the thickness of the air boundary layer at the leaf surface. We suggest that the smaller leaf sizes at the colder sites would constitute an adaptation to avoid frost damage, given the trend of large leaves with thick boundary layers to develop nighttime temperatures lower than air temperatures.