Résumé
Key Message: Needle anatomic plasticity enables fast-growing maritime pine trees to cope with water limitations by enhancing both water-use efficiency and embolism resistance.Abstract: Maritime pine is a major forest tree grown for wood production in Southern Europe. A breeding program for the selection of fast-growing varieties was established in the 1960s, in France. In the context of climate change, the magnitude of phenotypic plasticity is a key issue for the prediction of putative decreases in productivity in the improved genetic material. We characterized the phenotypic plasticity of anatomical and physiological traits in two families with contrasting growth rates, under different water regimes, at the juvenile stage. An analysis of 38 traits showed that the fastest growing family had the greatest phenotypic plasticity for morphological, anatomic, chemical and physiological traits, enabling it to increase its water use efficiency and embolism resistance in response to water deficit. The observed modifications to the extravascular (proportion of spongy parenchyma in needles) and vascular (xylem in the needles and stem) compartments in response to water constraints were consistent with a higher water use efficiency and greater embolism resistance. The ability to optimize meresis and auxesis according to environmental conditions during needle development could be related to growth performance over time in different environmental conditions. These results suggest that selection for growth in maritime pine leads to the selection of individuals with greater phenotypic plasticity related to higher performances in non-limited conditions.