Growth and ecophysiology
At the end of both cultivation periods, we counted all surviving plants to estimate mortality and measured the height of all surviving plants. On a subset of five plants per population we determined exchanges of CO2 and water vapour using an open flow system carrying air with ambient CO2 pressure (41 ± 1.5 Pa) from the Botanical Garden of Pavia University into an infrared gas analyser (LCA-4, ADC Co., Hoddesdon, UK) connected to a factory-designed small (625 mm2) leaf chamber. Illumination was provided by a glasshouse lamp at 450 ± 20 µmol m−2s−1 PPFD and ambient temperature (25 ± 2 °C). Each gas-exchange value was the mean of 3-5 measurements per plant. The gas-exchange measurements were used for determining area-based net CO2 exchange (net photosynthesis, as µmol CO2 m−2 s−1), stomatal conductance ( gs, as mol H2O m−2 s−1) and water use efficiency (WUE, as µmol CO2mol−1 H2O m−2s−1). Chlorophyll a fluorescence was determined in three mature healthy leaves by a portable fluorimeter (OS1-FL, Opti-Sciences, Hudson, NH, United States). Fm was obtained using a saturating pulse of actinic light after 1 min of acclimation under which conditions Fo mas measured. The ratio Fv/Fm [with Fv = (Fm – Fo)] was used to assess the photochemical efficiency of photosystem II as a proxy of stress experienced by the plants during the experimental cultivation. Chlorophyll (chl) content was determined (as arbitrary units) on a further subset of 5 plants with a chl content meter (CCM-200; Opti Sciences, Tyngsboro, MA, USA).