Age- and position-of-origin and rootstock effects in Douglas-fir plantlet growth and plagiotropism

Plagiotropic angle of seedling-derived Douglas-fir plantlets varied with position of adventitious bud origin on the explanted cotyledon rosette, being least at its center (18°) and greatest (45°) along the basal third of the cotyledon. When the tops of plagiotropic plantlets (55°) were grafted to seedling rootstock, they assumed a near-vertical orientation (10°), with pectinate changed to radial leaf arrangement, within 5 months. Conversely, seedling tops grafted to plagiotropic plantlet rootstock grew plagiotropically (56°). These, and other observations lead to a hypothesis that plagiotropism in cotyledon-derived plantlets results in part from an incomplete vascular connection of the root system to the shoot.In contrast, the greater plagiotropic angle in plantlets from a 12 year-old tree, decreased by only half (from 72° to 34°) after grafting to seedling rootstock. First-season height increment of these plantlets was only 60 percent of seedling or juvenile plantlet height increment, and was unaffected by rootstock type. The adult-origin plantlets exhibited mature shoot morphology, and unchanged plagiotropism after 2 years growth in large pots. Thus it appears that the culture-induced juvenile appearance and behaviour noted for this material when maintained in vitro, is dependent on the continued presence of the culture conditions.