Moving Waves of Bacterial Populations and Total Organic Carbon along Roots of Wheat

Abstract

To determine if spatial variation in soluble carbon sources along the root coincides with different trophic groups of bacteria, copiotrophic and oligotrophic bacteria were enumerated from bulk soil and rhizosphere samples at 2 cm intervals along wheat roots 2, 3, and 4 weeks after planting. There was a moderate rhizosphere effect in one experiment with soil rich in fresh plant debris, and a very pronounced rhizosphere effect in the second experiment with soil low in organic matter. We obtained wavelike patterns of both trophic groups of bacteria as well as water-soluble total organic carbon (TOC) along the whole root length (60 or 90 cm). TOC concentrations were maximal at the root tip and base and minimal in the middle part of the roots. Oscillations in populations of copiotrophic and oligotrophic bacteria had two maxima close to the root tip and at the root base, or three maxima close to the tip, in the middle section, and at the root base. The location and pattern of the waves in bacterial populations changed progressively from week to week and was not consistently correlated with TOC concentrations or the location of lateral root formation. Thus, the traditional view that patterns in bacterial numbers along the root directly reflect patterns in exudation and rhizodeposition from several fixed sources along the root may not be true. We attributed the observed wavelike patterns in bacterial populations to bacterial growth and death cycles (due to autolysis or grazing by predators). Considering the root tip as a moving nutrient source, temporal oscillations in bacterial populations at any location where the root tip passed would result in moving waves along the root. This change in concept about bacterial populations in the rhizosphere could have significant implications for plant growth promotion and bioremediation.