Research shows that the presence of water and nutrients affect plant belowground carbon allocation and root architecture within soil. However, many of the physiological traits involved in the uptake and assimilation of nutrients are not important for water acquisition (e.g., nitrogen reduction and conversion, ion transportation, pH control). We predicted that the architectural and morphological root traits involved in the acquisition of nutrients are likewise unimportant for water acquisition. To investigate the effects of resource availability on the root system, we grew Panicum virgatum in a split-root design that partitioned water and nutrients to separate sections of the root system. We hypothesized that plants would respond to the presence of nutrients by investing more carbon into root traits that enhance root surface area, whereas portions of the root system important for water acquisition may exhibit less surface area but greater root length and diameter (to promote water transport). By tracking root architectural development in the presence of a single plant resource (nutrients or water), we can link architectural traits in part of the root system to specific functions that roots undergo in the presence of that single resource.
Results/Conclusions
Panicum virgatum differentiated between nutrient-only compartments and water-only compartments by allocating a greater proportion of root length and volume into the nutrient-only compartment at the 0-0.25 mm and the 0.25-0.5 mm diameter classes. Plants allocated approximately equal proportions of root length and root volume to both compartments for diameter classes above 0.5 mm, which suggests that roots of a diameter greater than 0.5 mm are not important for nutrient acquisition and assimilation. The proportion of root tips allocated to nutrient-only compartments was greatest at the 0-0.25 diameter class and decreased with increasing diameter; however, differences in root tips per compartment were not as great as root length or volume. This indicates that Panicum virgatum is allocating more carbon towards root elongation in nutrient-only compartments but is not investing in the production of new lateral roots as much as was expected.