The mysterious root length

Abstract

reflecting the difficulty in extracting fine roots from the soil, and the enormous length these roots can attain. For example, Dittmer (1937) excavated the entire root system of a single rye plant, which consisted of 13,815,672 branches and had a total length of 622 km with a surface area of 237 m, and a total root hair length of 11,000 km. Since then, progress has been made in the methodology of assessing root length and in understanding the variation in it, but there still remains an air of mystery around root length as even the newest findings are often contradictory. Investigations have often focused on Specific Root Length (SRL), a trait which characterizes the economical aspects of a root system, stating the costs––mass––per potential return––root length. SRL has been shown to increase, decrease, or stay constant in response to nutrient limitation (Ryser 1998). Furthermore, root length does not always respond to specific ions, such as nitrate or phosphate, in manner like theories based on the mobility of these ions would predict (Robinson 1996; Leyser and Fitter 1998). In this issue, Hill et al. (2006) describe an elegant experiment to tackle this mystery. They investigate the variation in specific root length and root surface area in response to variation in N and P supply among 10 temperate pasture species with contrasting root morphologies and contrasting N and P requirements. They investigate the traits contributing to variation in root surface area: biomass allocation to roots, SRL, root fineness and tissue mass density. The results show that the root morphology of a species indeed corresponds well with its phosphorus requirements. The data also clearly illustrate the general importance of phenotypic plasticity for the maintenance of resource acquisition capacity at low levels of the resource, as all species were able to maintain a fairly constant root length across a wide range of P treatments, despite a large variation in total plant mass and root mass. However, the degree of the plastic response was not related to the P uptake capacity of the species, and the patterns cannot readily be generalized. In response to P limitation, some species changed their biomass allocation pattern, some changed root fineness, and others root tissue mass density. There was no consistent relationship in the nutrient level at which the species began to modify their root characteristics and their nutrient requirement. P. Ryser (&) Department of Biology, Laurentian University, Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6 e-mail: pryser@laurentian.ca Plant Soil (2006) 286:1–6 DOI 10.1007/s11104-006-9096-1