## 1 Introduction
Micro and nano bubble generators have recently been introduced into bioreactor design [48], inspired by technologies for the delivery of oxygen and ozone in wastewater treatment [49]. The large surface of micro/nano bubbles as compared to conventional bubbles improves oxygen transfer efficiency. The drawback of micro and nano bubble generators is that they require high-pressure pumps. Furthermore, in airlift mode the liquid velocity decreases with bubble radius for bubbles smaller than 100 \(\upmu\)m [50]. Because micro bubbles are smaller than 50 \(\upmu\)m and nano bubbles are smaller than 200 nm [49], micro bubbles and especially nano bubbles are less suitable to drive liquid transfer in airlift bioreactors. Small-scale inexpensive bioreactors for screening purposes and comparative studies are lacking, limiting our ability to address differences in the exudation potential of hairy root cultures derived from different plant accessions. Studies of the relationship between genetic polymorphism and secondary metabolite accumulation in plant tissues e.g., [34; 51; 52] and in root exudates [53] calls for comparative analysis of numerous plant accessions. Exclusion of the effect of bacteria and fungi colonizing plants as endophytes, pathogens, and inhabitants of the phyllosphere require axenic conditions, which can most conveniently be achieved in sterile hydroponic cultures. Lack of small and economical incubators for sterile hydroponics limits these studies. Our laboratory studies the chemical diversity in sesame [34], extending the focus from seeds to roots and hairy roots and their exudates. In another project we study biological functions of salicylic acid, which may fulfill different roles in different species and even in different tissues of the same plant species. Salicylic acid is known to act as a signal of plant defense response in plant shoots [54] but we also found it in xylem sap of plants infected with a fungal pathogen [55] and in root exudates of healthy plants, where it facilitated mobilization of soil phosphorus [1]. Salicylic acid was also reported to participate on shaping microbial communities of the rhizosphere [56], serving as a signal in interorganismal interactions. Comparative analysis of salicylic acid exudation by different plant accessions requires growing multiple sterile cultures in parallel. In another project in our laboratory we study replant disease of roses. This project relies on the analysis of root exudates because we pursue a hypothesis explaining the disease by the effects of certain components of root exudates accumulating in the rhizosphere of roses [57]. Root exudates of rose accessions known to suffer from replant disease have to be collected under sterile conditions, their components identified and the phytotoxicity of these components determined. Progress in all three projects has been hampered by the lack of small bioreactors for sterile hydroponics and hairy root cultures. The objective of this work was to develop a small-volume, low-cost bioreactor and validate its suitability for hydroponic cultures of _Brassica napus_ and _Rosa canina_, for growing hairy roots of _Hyoscyamus niger_ and _Sesamum indicium_ and for the collection of root exudates and hairy root exudates suitable for HPLC analysis.