Poplar Bark Storage Proteins (BSP) are important in storing nitrogen (N) recovered from autumn-senescent leaves, and supply N for spring growth. Plants of Populus tremula x P. alba were transformed with a poplar BSP antisense cDNA fused to a double 35S promoter. Regenerated lines were screened for reduced BSP accumulation in short day photoperiod, and two lines were selected for further studies. Under long-day conditions, both BSP-antisense lines were characterized by larger leaves, longer internodes and slower growth rates, for a net shift in dry-matter partitioning from stem to leaf. Antisense lines also show reduced N concentration in leaf and stem tissue, and altered nitrate uptake. Grafting studies show that these effects on leaf and stem partitioning and N content are determined by the shoot genotype. These results indicate an important role of BSP in long day growth and partitioning. BSP-antisense plants did not show altered Nitrate Reductase activity, as determined by in vivo assay. Wild-type poplar plants were grown hydroponically on solutions of 0 to 30 mM nitrate, and NR activity determined on leaf, stem and root tissue. Leaf activity was >20x higher than root or stem, with the highest activity found in young expanding leaves. NR activity of both leaves and roots increased with N supply. During autumn, BSP-antisense leaves abscised earlier than the untransformed wt. Abscised wt leaves contained a higher proportion of pre-senescent N levels. Comparisons among ecotypes of P. deltoides and among clones of P. trichocarpa x P. deltoides demonstrated genetic variation in both time of BSP induction, and amount of BSP accumulation. In six P. deltoides ecotypes grown at a common site, time of maximum BSP mRNA was inversely correlated with latitude of origin. Eight to ten clones from each of six full-sib families of P. trichocarpa x P. deltoides were screened for SD BSP accumulation. Clonal differences in BSP accumulation were significant in 5 of 6 families, and clones with high BSP levels also had higher total stem N content. These results further confirm the importance of BSP in autumn N resorption. / Graduation date: 2000