Abstract

The economics of plant root building show predictable relationships with root growth, death, and nutrient uptake strategies. Plant taxa with inexpensively constructed roots tend to explore nutrient hotspots more precisely than those with expensive constructed roots, but at the cost of more frequent tissue turnover. This compromise underlies an acquisitive to conservative continuum in investing in resources, described as the “root economic spectrum (RES)”. Yet the adaptive role and genetic basis of RES remain largely uncertain. Different ecotypes of switchgrass (Panicum virgatum) exhibit root characteristics exemplifying SER, with expensive built roots in the southern lowlands and inexpensively built roots in northern highland ecotypes. We used an outbred genetic mapping population derived from lowland and highland switchgrass ecotypes to examine the genetic architecture of RES. We found that the absorbing roots (distal first and second order) were often “deciduous” in winter. The percentage of winter-absorbing roots was decreased by northern highland alleles compared to southern lowland alleles, suggesting a conservative strategy adapted locally in a warmer and acquisition strategy in colder regions. The relative turnover of absorbing roots was genetically negatively correlated with their investment in biomass per unit root length, suggesting that the key trade-off in the regulation of RES is genetically facilitated. We also detected strong genetic correlations between root morphology, root productivity and shoot size. Overall, our results reveal the genetic architecture of several traits that likely impact the evolution of RES and the air-subsoil organization of plants. In practice, we provide genetic evidence that increasing the yield of switchgrass for bioenergy does not directly conflict with improving its sequestration of carbon derived from the roots.