Studies of adventitious rooting in forest trees, relative to studies of other traits in genetics literature, are published at a similar frequency as in agricultural and horticultural crops. Unlike crops, however, the desire to manipulate vegetative propagation rates has driven research in forest trees. Until recently, the role of adventitious rooting as an adaptation to water-logging, nutrient deficiency stress, or in understanding biomass allocation within a tree, has been a not been a motivator for genetics research. Studies span the major economic taxa, Pinus, Populus and Eucalyptus spp. with some studies in Quercus and Salix, and with most work focusing on rooting of stem cuttings rather than in vitro propagation. This chapter will discuss what is known about the genetic control of adventitious rooting in forest trees and how this knowledge is implemented in breeding programs designed to improve specific traits of trees and deployment of hybrids in clonal forestry programs.
Quantitative genetics studies of adventitious rooting of stem cuttings in forest trees have shown that AR traits are typically under moderate genetic control providing a basis both for selection for improvement and for more detailed understanding of genetic control with molecular approaches. Both additive and non-additive genetic effects have been reported, but it is likely C-effects have led to inflation of broad-sense heritability estimates in particular. A spectrum of genetic architectures have been proposed for adventitious rooting in forest trees, but oligogenic architectures and major Quantitative Trait Loci (QTL) predominant in interspecific hybrids. Oligogenic architecture is evident across genera, Eucalyptus, Pinus and Populus, and mirrors the large effects detected for other traits of adaptive significance observed in advanced generations of hybrids of trees and other plants.
There has been an emphasis on interspecific hybrids in quantitative trait loci studies concerning forest trees because of the importance of adventitious rooting to the vegetative propagation, which is often vital to the success of hybrid production. Hybrids, when combined with clonal propagation, are an attractive experimental system because they offer increased power for quantitative trait loci detection. Additionally, large segregating effects are both predicted and detected in advanced generation hybrids between species divergent for AR traits. Combined, these factors have created a fertile niche for quantitative trait loci studies of adventitious rooting in forest trees. The rising importance of advanced generation hybrids, and the efficiencies expected for markeraided selection, also arguably make quantitative trait loci studies of adventitious rooting of foremost practical significance because of their direct relevance to the populations under improvement and deployment.