Altered fibre development in petioles and stems of transgenic UBI::PtaHB1 poplars. (A) Mean (± SD) ratio of fibre vessel elements determined from image analyses based on four separate petioles for two transgenic lines carrying the UBI::HB1 construct and one wild-type differed significantly according to Student's t test (p < 0.001). (B) Cross-sections of mature petiole (40×, LP 21), observed under UV-illumination to reveal autofluorescence of lignified cell walls of fibre and vessel elements. (C) Distribution of fibre lengths in mature petioles (LPI 21) and partially lignified stems (LPI 6) determined by FQA from an average 8000 cells per sample. Each histogram bar represents the average proportion (%) of cells in a given length class (or bin) from two transgenic lines (three plants each) and six wild-type plants. Numbers in brackets are the number of fibres counted for selected bins. (D) Average fibre length determinations at three stages of development (LPI 6, LPI 21, LPI 43). Bars indicate average length ± SD of 6 samples analyzed for each treatment. * indicate fiber counts (D, whole population or C, bins) that differed significantly between transgenic and control trees (one-way ANOVA at p < 0.05).