Relative expression rates of the target genes APX and SOD. Relative gene expression rates of APX (A) and SOD (B) in roots of the poplar clones 58-861 and Poli were determined. Expression was calculated using all tested approaches for relative quantification, normalized to ACT, EF1 and UBQ transcript abundance and expressed in relation to WW Poli as calibrator sample. ddCq, comparative Cq method; L&S aEff, Liu & Saint method using average amplification efficiencies for each gene; L&S iEff, Liu & Saint method using individual amplification efficiencies for each reaction. ACT, actin; APX, ascorbate peroxidase; EF1, elongation factor 1; SOD, superoxide dismutase; UBQ, ubiquitin; WW, well-watered; WL 75%, water limited (75% less water than control), n = 3. Means ± standard errors indicated by the same letter are not significantly different (P ≥ 0.05) for each of the tested quantification approaches.