Fig 4.

(a) Combined data from the hydroponic experiments (Experiment 1 and Experiment 3) illustrate an overall significantly greater ¹⁵N incorporation by dosed wild poplar plants compared to undosed plants (p < 0.001). In vitro propagated Nisqually-1 plants showed an intermediate response that was significantly lower than dosed wild plants (p < 0.05) while only marginally greater than undosed wild plants (p = 0.08). In these experiments, rooted cuttings were exposed to ¹⁵N₂ gas in N-free hydroponic medium (NFM) for two weeks. Bars annotated with the same letters are not significantly different from each other based on pairwise comparisons of least squares means at alpha = 0.05. Error bars represent one standard error. (b and c) Distributions of δ¹⁵N‰ data from dosed wild poplar cuttings tested in Experiment 1. The percent frequency plot (b) shows that about a half of dosed wild cuttings incorporated ¹⁵N at similarly low levels as the in vitro grown Nisqually-1 cuttings from Experiment 3 (vertical dashed line). The box plot of δ¹⁵N‰ grouped by source trees (c) suggests that the capacity to harbor N₂ fixing endophytes is likely to vary not only between trees but also between locations within a single tree.