Sun and shade leaves of Olea europaea respond differently to plant size, light availability and genetic variation

1. Canopy plasticity, the expression of different leaf phenotypes within the crown of an individual tree has complex functional and evolutionary implications that remain to be thoroughly assessed. We hypothesized that it can lead to disparity in how leaves in different positions of the canopy change with allometric growth and population genetic structure. 2. Leaf phenotypes of the inner and outer canopy were estimated using eight morphological and physiological characters. All traits were measured under field conditions in six populations of Olea europaea and again in a common garden for a subset of the genotypes. The same populations were characterized genetically with amplified fragment length polymorphisms (AFLP) genomic scans. With these data, we investigated the extent to which leaf phenotypes change with plant size, genetic processes and in response to environmental conditions inside and outside the canopy. 3. The size of trees measured in the field was clearly associated with the phenotype of sun but not to that of shade leaves. The phenotype of sun leaves depended on both direct and diffuse light, while that of shade leaves was found to correlate only with diffuse radiation. Additionally, light availability inside the canopy was conditioned by the shape of external leaves, and increasing elongation of sun leaves led to higher radiation in the inner canopy. 4. The field phenotypes of both inner and outer canopy leaves were correlated with genetic variation among populations. Conversely, in the common garden, the different genotypes expressed a homogeneous sun phenotype, while phenotypic differences among populations remained apparent in shade leaves. 5. We conclude that, in agreement with our working hypothesis, canopy plasticity is both cause and consequence of the environment experienced by the plant and might lead to the differential expression of genetic polymorphisms among leaves. Furthermore, we propose that it can contribute to buffer abiotic stress and to the partition of light use within the tree crown.