Phenotypic plasticity of Quercus ilex L: relationship between seed size, germination and seedling growth phases.

Phenotypic plasticity is the capacity of a given genotype to express different phenotypes in different environments. Plasticity for physiological and life-history traits may allow plant species to grow and reproduce in spatially or temporally variable environments; moreover, the timing of plant development can itself be plastic. The main objective of this research was to analyse phenotypic plasticity of Quercus ilex L. Seedlings were germinated from seeds of parent plants from three different localities in Italy: Nago, in the Garda Lake region, at the Northern limit of holm oak distribution area in Italy; Castelporziano, near Rome, at sea level, at the centre of the distribution area; and Frassanito, near Lecce, at sea level, in a dry area of Southern Italy. Q. ilex is an evergreen sclerophyllous species widely distributed in the Mediterranean Basin; thus, knowledge concerning individuals from different climates is important to forecast the species potential productivity to increasing drought stress. The results of the present research underline the relationship between seed size (length, diameter, mass) and the germination rate. Larger seeds from the sites characterised by a higher drought stress (southern origin) have a higher germination rate than smaller ones (northern origin), characterised by a higher cold stress. Moreover, the results underline the relationship between seed size and seedling traits of the considered ecotypes: in particular, seedlings from larger seeds have a higher relative growth rate (RGR) than those from smaller seeds, especially during the first developmental stages, so that they may be provided with sufficient reserves to grow before air temperature and soil water are not limiting factors. In fact, the highest RGR in the first seedlings developmental stages allows the highest seedling height, leaf area, and total leaf area per plant. The effect of seed size could be of adaptive significance in establishing and maintaining populations in natural environments. Moreover, the response of the considered ecotypes to experimentally imposed water stress underlines that seedlings from Frassanito seeds, evolved under naturally prolonged drought stress, seem to be more tolerant to water stress and they might provide a substantial advantage in drier sites. After the imposed water stress, the calculated leaf damage is the greatest in the ecotype evolved in the centre of the distribution area, characterised by the lower drought and cold stresses. The plasticity index for physiological Q. ilex leaf traits is higher for physiological traits than for morphological and anatomical ones. Because plasticity influences environmental tolerance, different plastic responses may contribute to differences in the range of environments that species inhabit, and in the specific capability to maintain functioning in contrasting conditions, including global change.