Current techniques and prospects for the clonal propagation of hardwoods with emphasis on Eucalyptus.

Abstract

Mass vegetative propagation has become an important tool for increasing the competitiveness of the forestry based industry. This method reaches its highest potential when it is used to establish clonal forests of hybrids endowed with better wood quality and higher volumetric growth. However, in several hardwood species, most notably in Eucalyptus, the popular method of rooting stem-cuttings has limitations, for example, rapid loss of rooting competence due to ontogenetic aging, intra-clonal variation resulting from topophysis, and poor quality of the root system, that in combination negatively affect genetic expression of some clones. Two alternative super-intensive systems, one micro- and one mini-cutting, for cloning Eucalyptus at a commercial scale have been applied successfully. These systems have shown great potential for offering technical and economic advantages not available from conventional stem cuttings. The micro-cutting system uses the apices obtained from micro-propagated plantlets, while the mini-cutting system is based on the rooting of axillary shoots from rooted stem-cuttings. In both systems the plants are managed intensively to produce small cuttings. Field clonal hedges are replaced by indoor hydroponic mini-hedges, which provide plantlets or rooted cuttings with a high degree of juvenility. The success of the system is also dependent on achieving optimal nutrient status in the resulting mini-cuttings. Compared to stem-cuttings, the micro- or mini-cuttings systems have improved rooting potential, rooting speed, and root system quality, as well as reduced costs. Additionally, these systems offer propagules with increased uniformity, and greatly reduced topophysis effects. The development of these super-intensive cloning systems has set the stage for a new phase of mass vegetative propagation of Eucalyptus and other hardwood species. Another important tool for mass propagation of hardwoods at high multiplication rates is in vitro somatic embryogenesis, a technique whose detailed understanding can benefit from new methods of gene expression analysis. Better regeneration rates are required for the commercial viability of most hardwood somatic embryogenesis current procedures. As gene delivery and transformation protocols for tree species improve, embryogenesis methods will grow in importance, since they create suitable target tissues for gene transfer. Nonetheless, at the core of every clonal propagation technique, lies the process of adventitious rooting. In spite of concerted efforts in the last three decades to fully understand the mechanisms of this developmental process, relatively little is known of the exact underlying steps that lead to the formation of a new root system in cuttings. Significant progress has derived from detailed physiological studies, the analysis of rooting mutants, and Agrobacterium rhizogenes-induced rooting. More recently, the global analysis of rooting-associated gene expression using DNA arrays has proven a very promising methodology to analyze complex developmental events. It is expected that new commercial clonal propagation protocols for hardwoods will emerge from some of these basic studies, as the understanding of the genetic control of adventitious rooting improves.