Metabolites, their decomposition, production of tomato and bioassays from open and closed rockwool systems

Growth, decrease in yield and other, undefined problems are reported to be due to the recirculation of the nutrient solution, which is compulsory for crops grown on substrates in the Netherlands. In a trial with tomato grown on rockwool, open and closed systems were compared. Drain water from both systems were analysed by GC-MS and HPLC for selected metabolites. Nutrient solution from the rockwool slabs was tested for the ability to decompose caffeic acid. Drain water from the trial and nurseries was tested in bioassays. The open and closed systems produced equal yield at a level of 50 kg m-2 and comparable with Dutch horticultural practice. GC-MS and HPLC showed that drain water contained fatty acids, iso-alkanes and some other metabolites. The concentrations of phenolic acids were low (most of them less than 0.1 µM) and the ligand of the DTPA-Fe-chelate did not accumulate. An unidentified peak in the HPLC-chromatogram could be an indication that the ligand decomposed and formed a product. Caffeic acid, added to the solutions sucked from the rockwool slabs, was decomposed within three days, but not in sterilised solutions. The bioassays were not negatively affected by drain water from the tomato trial. In contrast, problems with the growth of tomato have been found with the drain water from two nurseries. It was concluded that the problems were caused by decaying roots from the seven years old rockwool slabs and from fungicide dripping from the plants into the drain water collection system.