Significant advances in the analyti

Significant advances in the analytical sector allow us to move from targeted compound analysisto root exudate fingerprinting, thereby providing novel in-depth information on the compoundcomposition in its entire complexity that triggers many rhizosphere processes (for more detailssee the review paper on metabolomics by Bouwmeester and van Dam in this issue). However,the impact of high-end analytical approaches is determined by our ability to identify relevantcompounds as well as by our exudate sampling techniques. To avoid alterations in the exudationprofile as a result of sorption to the soil matrix and microbial decomposition, the majority ofstudies on root exudates have been and still are carried out in hydroponic systems. While thealtering effect of non-sterile conditions on the exudation profile is well investigated [14], the few comparisons between soil and nutrient solution culture as a growth medium showed a signifi-cant, soil-dependent difference in compound concentrations released by roots [9,15]. Althoughhydroponic set-ups allow (relatively) simple collection of root exudates, the relevance of obtainedresults remains questionable with respect to field conditions.