Shoot position affects root initiation and growth of dormant unrooted cuttings of Populus

Rooting of dormant unrooted cuttings is crucial to the com mercial deployment of intensively cultured poplar (Populus spp.) plantations because it is the first biological prerequisite to stand establishment. Rooting can be genetically controlled and subject to selection. Thus, our objective was to test for dif ferences in rooting ability among cuttings from three positions on cutting orchard plants of five genomic groups ([Bartr. ex Marsh x P. trichocarpa Torr. & Gray P. deltoides] x P. deltoides 'BC', P. deltoides 'D', P. deltoides x P. maximowiczii A. Henry 'DM', P deltoides x P nigra L. 'DN', P. nigra x P. maximowiczii 'NM'). Cuttings, 20 cm long, were randomly planted at 1.2- x 2.4-m spacing across three planting dates during 2001 and 2002 at Ames, Iowa, USA(42.0°N, 93.6°W); Waseca, Minnesota, USA (44.1°N, 93.5°W); and Westport, Minnesota, USA (45.7°N, 95.2°W). We measured root dry weight, number of roots, and total root length from harvested cuttings after 14 d of growth. Rooting traits varied relative to stem position but interactions of genomic groups and positions and genotype x environment interactions existed on multiple-year and single-year bases. Position accounted for the second highest amount of variation (≥ 5%) for all rooting traits. Cuttings from the basal third of the shoot system of the stool plant exhibited nearly two times more rooting as those from middle and apical regions, whereas middle cuttings exhibited similar rooting trends as apical cuttings, for all rooting traits. The percentage of cuttings rooted across years was greatest with basal cuttings for the BC, D, DM, and DN genomic groups (> 50%). Middle cuttings of the NM group survived at a greater rate (88%) than did basal (80%) and apical (72%) cuttings. Single-year analyses of interactions of genomic groups and positions showed rooting was greatest with basal cuttings for BC, D, and DN genotypes. Basal cuttings of the DM and NM genomic groups did not clearly outperform middle and apical cuttings, and differences among all cutting positions were site- and year-dependent.