Altered petiole development in UBI::PtaHB1 transgenic poplars. (A) Six-month-old plants of the wild-type (WT) and UBI::PtaHB1 (representative of transgenic lines from three independent transformation events). (B) Close-up view of mature petioles to show angle relative to the main stem. (C) Distribution of petiole lengths (i) from the first fully expanded leaf (inter-node position: LPI 0) to the last healthy leaf (approximately LPI 50). Mean length (ii) was calculated from LPI 8 to LPI 45 (38 internodes, n = 228) and Student's t test was applied to the data from each internode separately (n = 6 per class; p < 0.05) Histogram bars represent average values (cm). (D) Distribution of petiole angle (i) from the first mature leaf below the area of stem elongation (LPI 8) to the last healthy leaf at the bottom (approximately LPI 50). The mean angles (ii) were calculated in the same manner as mean length. Open circles are used for the wild-types and closed circles for the PtaHB1 transgenic (C, D).