The rooting capacity of microshoots derived from two mature Eucalyptus urophylla X Eucalyptus grandis half-sib clones kept for 3 y under intensive micropropagation was assessed in different in vitro conditions. A first set of experiments established that clone 147 microshoots rooted earlier and in greater proportions, while producing more adventitious roots overall than their homologs from clone 149. Modifying the composition of the basal 1/2-MS-derived rooting medium by 1/4-MS or Knop macronutrients, or reducing sucrose concentration to 10 g l−1 did not enhance the rooting rates. However, together with the growth regulators added, they had a significant effect on the number of adventitious roots formed. With rooting rates reaching 81%, the higher rootability of clone 147 over clone 149 was further confirmed by the second set of experiments with significant effects of the various auxins tested and strong clone × auxin interactions on the proportions of rooted microshoots and on the number of adventitious roots. The best rooting scores were given by 5 μM indole-3-butyric acid (IBA) and 12.5 μM 1-naphthaleneacetic acid (NAA), whereas the microshoots exposed to 5 or 12.5 μM indole-3-acetic acid (IAA) were less responsive. Lower light intensities did not improve significantly root capacities, although differences might exist according to the genotype. Overall, root and shoot elongation was stimulated by light. At the end of the experiment, the rooted microshoots were markedly taller than the non-rooted ones, with significant influences of auxins and light intensity, and to a lesser extent, of the genotypes.
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Vol. 44 • No. 4