Abstract
Differences in rooting ability of birch (Betula pubescens J.F. Ehrh.) cuttings were observed as a result of differences in genotype and physiology of the stock plants. The uniformity in response among cuttings from micropropagated plants compared with cuttings from seed plants confirmed the advantage of using micropropagated plants to study environmental effects. Shoot morphology of the seed stock plants was influenced by both photoperiod and thermoperiod. A day/night temperature of 15/25 degrees C reduced stem elongation compared with a day/night temperature of 25/15 degrees C regardless of photoperiod, and a continuous light regime resulted in more shoots per plant in both temperature regimes than a 16-h photoperiod. A reduction in the supply of macronutrients did not influence shoot morphology, but increased rooting substantially and seemed to override the effects of environmental factors. In cuttings of seed plants, the highest rooting percentage and number of roots were obtained in a 16-h photoperiod with a day/night temperature of 15/25 degrees C. In micropropagated stock plants, there was a positive correlation between shoot length and number of leaves per shoot and topographical distribution of light within the plants, but there was no correlation between these parameters and rooting ability of the cuttings. A rooting temperature of 16 degrees C delayed the rate of root production compared with the rate at higher temperatures, but the final rooting percentage was the same over the range from 16 to 28 degrees C. Root branching increased with temperature. At all temperatures, there was a large increase in sucrose content at the base of the cuttings during rooting, whereas the concentration of nontranslocated sugars remained constant. The carbohydrate content at the base of cuttings from micropropagated stock plants was three times higher than at the base of cuttings from seed stock plants, but the higher carbohydrate content was not correlated with a higher rooting potential.