Improved growth in a field trial of transgenic hybrid poplar overexpressing glutamine synthetase.

•  In the present work the performance of transgenic poplars expressing a pine glutamine synthetase (GS) transgene was studied in natural conditions. •  A field study of eight independent transgenic lines and control plants was carried out for 3 yr in the province of Granada (Spain). •  Transgenic poplars reached average heights that were 21, 36 and 41% greater than control plants after the first, second and third year of growth, respectively. Transgene expression affected plant features with time resulting in increased protein, total GS and ferredoxin-dependent glutamate synthase (Fd-GOGAT) in leaves. However, neither differences in the large subunit of Rubisco (LSU) abundance nor water content were detected between lines. Furthermore, no significant differences were found in total polysaccharide and lignin content in tree trunks. •  The analyses of stem diameter, and protein contents in the bark suggest that higher levels of nitrogen reserves accumulated in the stem of transgenics. Our results suggest that modification of GS1 expression may be a useful strategy to complement traditional tree breeding in short rotation plantations.