The importance of a sterile rhizosphere when phenotyping for root exudation

A fast and reliable phenotyping system forms a major bottleneck in root exudation research. Our goal was to develop such a system to quantify genetic and environmental effects on root exudation. Another aim was characterizing non-sterile phenotyping. We developed a system in which plants can be grown with non-sterile shoot environments and sterile rhizospheres. These sterile systems were compared with non-sterile controls. In non-sterile rhizospheres exogenous carbon disappeared quickly with a half-life of 2 to 3 h and root exudate concentrations remained below detection limit. In sterile rhizospheres exogenous carbon levels were relatively stable or depleted slower than in non-sterile rhizospheres and organic acid build-up occurred. Tomato (Solanum lycopersicum) could be grown with sterile roots for several months. The differences in carbon depletion in sterile and non-sterile rhizospheres was most likely due to the absence of microbial catabolism in sterile rhizospheres. These results prove that using a sterile phenotyping system is essential to study the quantity and composition of root exudates. The sterile system described in this paper eliminates the obscuring effect caused by microbes on exudation levels. It offers a stable, reliable and easy phenotyping method and can be used to investigate genetic and environmental effects on exudation.