Root Growth Plasticity of Hybrid Poplar in Response to Soil Nutrient Gradients

Abstract

Forest trees are often intensively selected and bred for yield improvement, yet most of this effort has been devoted to shoot rather than root traits. Genetic variation exists, but has not been exploited, in a variety of tree root traits such as specific root length (length of root per dry weight, Eissenstat 1991), vertical distribution (Heilman et al., 1994), horizontal extent of roots (Friend et al., 1991), and root growth plasticity in response to nutrient-enriched soil patches (George et al., 1997). Root growth plasticity, the increase in root growth associated with enriched zones (or patches) of soil, is of particular relevance 134because it may enhance the efficiency of root investment into nutrient acquisition in heterogeneous soils. We explored the variation in root growth plasticity in Populus hybrids. P. trichocarpa Torr. & Gray × P. deltoides Marsh, hybrids are some of the highest-yielding trees in the world, with total biomass productivity of up to 35 Mg ha−1 y−1(Scarascia-Mugnozza et al., 1997) and they have been the focus of intensive genetics and physiological studies. A three-generation pedigree of parentals, F1 hybrids, and F2 offspring have been particularly useful for quantifying gene sequences responsible for physiological traits (Bradshaw, 1996). We studied this pedigree because of its demonstrated variability and because of the potential to contribute root traits to existing selection and breeding schemes. The objective of this study was to quantify the variation in fine root growth of hybrid poplar in response to nutrient-enriched patches of soil and make some informed speculation about the application of this variation to yield physiology and poplar ideotypes.