Environmental control of adventitious rooting in Eucalyptus and Populus cuttings

Concerted control of irradiance, temperature, water availability, mineral nutrition and beneficial root-associated microorganisms significantly improves adventitious rooting in eucalypts and poplars, essentially by modulating auxin and carbohydrate metabolism. Eucalyptus and Populus are among the most economically relevant tree genera. Clonal propagation allows fast genetic gain obtained using elite genotypes. Adventitious rooting (AR), a complex and multifactorial process, is often the main limiting factor for tree cloning. Herein, practical and basic approaches to optimize clonal propagation of eucalypts and poplars, focusing on the main environmental control factors affecting it, are explored. Auxin homeostasis and function are central to AR. Irradiance quality and quantity, as well as temperature, can effectively modulate auxin availability, transport and activity. The interaction of carbohydrates, irradiance and temperature is also at the core of AR. Root architecture may be effectively modified by different N sources. Several macro and micronutrients impact on central factors of rhizogenesis, including energy metabolism, gene expression and enzymatic activities regulating auxin and other phytohormonal steady-states driving AR. Appropriate mineral nutrition is often determinant for successful AR and survival. Microbial associations with the root system and the rhizosphere, both bacterial and fungal, can have a role in auxin availability to cuttings, as well as improve disease resistance, nutrition and water relations. Significant cost reduction in clonal propagation systems of eucalypt and poplar are attainable with an adequate control of environmental factors, particularly for donor plants. Future studies should extend the molecular and physiological findings of basic research to the commercial propagation systems, and these, by their turn, should be explored to provide further advances in the basic understanding of this crucial developmental process for human economy.