Phenotypic plasticity in mesic populations of Pinus pinaster improves resistance to xylem embolism (P50) under severe drought

The objectives of the study were to assess the phenotypic variation in the vulnerability to water stress-induced cavitation (estimated by P₅₀, or the xylem water potential which causes a 50% loss of conductivity) and the trade-offs between P₅₀ and related hydraulic traits, i.e., stem specific conductivity (K _s), slope of the vulnerability curve (slope), wood density and branch size. Variability was examined for six Pinus pinaster populations covering the latitudinal range of the species and plasticity was tested through two provenance-progeny trial sites (xeric/mesic). As expected, the overall values of P₅₀, K _s and branch size decreased in the xeric site. Variation in P₅₀ and K _s among populations was mainly the result of phenotypic plasticity, while wood density was genetically controlled and not affected by the environment. Stress conditions in the xeric site promoted a convergence in P₅₀ and K _s as a result of the high phenotypic plasticity of the populations from mesic origins. In the mesic site, the ranking of populations for cavitation resistance and hydraulic capacity was consistent with the geographic location of the seed source. Higher resistance to cavitation was related to lower K _s, branch size and slope, mainly at the population level, but also as a general trend across individuals. In a warmer and drier climate, there could be a potential selection of Pinus pinaster populations from mesic origins, which showed a great responsiveness and adjustment to drought conditions (similar or higher P₅₀ than the populations from dry origins), in addition to a high wood density and growth.