Growth inhibition caused by reused drainage water; quest for cause and measuring method

Growth inhibition is often a reason for growers to refresh the recirculation water and to discharge the excess water. The cause of growth inhibition other than a high concentration of sodium or a disease is usually unclear. Objectives of the study were the determination of the cause of growth inhibition and the development of methods to determine growth inhibition in an early stage in the crop in situ without the use of more indirect bioassays. Potential growth inhibitory components in drain water of a rose cultivation have been investigated by means of stepped analysis in the crop, the substrate and in water. The course of the oxygen concentration in the root environment was monitored. The efficiency of photosynthetic light utilization had been measured with the aid of chlorophyll fluorescence on plant leaves in the crop. All the measurements were intended to determine a possible relationship between growth inhibition, photosynthesis and oxygen stress. The main results were: Water disinfec¬tion methods eliminated growth inhibition. Drainwater was filtered with filters of 0.2 and 0.45 µm. This resulted in a relative growth in the bioassay compared to untreated of 122% and 104%. Growth inhibition in water in the rose crop is probably caused by a microbiological factor with a bacterial origin. A mix of pesticides as used on com¬mercial nurseries has been tested with bioassays on cress seed. A 1, 10 and 100-fold concentration compared to the practical situation were tested. Pesticides in the drainage water in concentrations as used on commercial nurseries did not inhibit growth in the bioassays. Monitoring of the oxygen concentration in the water and root environment and the measurements of the light use efficiency showed no relationships with demonstrated growth inhibition. The value of these methods could not be assessed because growth inhibition was only found in conducted bioassays with drainage water and not in the crop.