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The physiological mechanism of water use efficiency (WUE) remains elucidated, especially in poplar. We studied WUEi (instantaneous leaf transpiration efficiency), WUEL (ratio of unused biomass and water), carbon isotope composition (δ13C), photosynthetic rates (Pn), stomatal density and stomatal conductance (Gs) in three different, randomly chosen Populus×euramericana clones: DN-2 (Populus × euramericana), R-270 (Populus×euramericana) and NE-19 (Populus nigra × (Populus×euramericana)) under well-watered conditions. The clones had great differences in WUEL, WUEi, δ13C, stomatal density and Gs. We concluded that, stomatal control was a key factor leading to Pn and WUEi differences and ultimately inducing WUEL and δ13C differences. δ13C is therefore, a good indicator to use when evaluating WUEL in Populus×euramericana. We also went further to clone the ERECTA gene, which regulates plant transpiration efficiency in Arabidopsis from Populus×euramericana. Real-time polymerase chain reaction amplification (PCR) analysis revealed that, PdERECTA may play the same role in Populus×euramericana as it does in Arabidopsis.
Key words: Carbon isotope composition, Populus×euramericana, PdERECTA gene, stomata, water use efficiency.