Root turnover: An important source of microbial substrates in rhizosphere remediation of recalcitrant contaminants

The growth dynamics and phenolic content of mulberry (Morus sp.) fine roots(<1 mm diameter) were determined and examined in relationship to rhizosphere remediation of recalcitrant soil contaminants. Root turnover measurements of rhizotron-grown plants showed that 58% of the fine roots produced during a 6-month growing season (June-November) died at the end of the season. The concentration of phenolic compounds in fine roots increased approximately 2-fold during the later stages of the season, and the total phenolic content of dead fine roots reached a maximum value of 38 mg/g dry weight. The late-season increase in total phenolics was primarily due to accumulation of three different flavones (morusin, morusinol; and kuwanon C). These three flavones were shown to support the growth of the bacterium Burkholderia sp. LB400, a degrader of polychlorinated biphenyls (PCBs). Thus, it has been established that, upon death, the fine roots of mulberry can serve as a source of substrate for PCB-degrading bacteria. These results establish for the first time that the chemical content and turnover rate of fine roots should be considered an important aspect of rhizosphere remediation.