Open access publications in the SLU publication database

Abstract

Recirculation of nutrient solution in greenhouse growing systems inhabits environmental and economic advantages regarding saving water and fertilizers. The disadvantages are a greater risk for spread of phytopathogens by the nutrient solution and for accumulation of organic compounds at phytotoxic levels. Organic compounds are excreted as root exudates by the crop as well as by microorganisms in the rhizosphere and are also being released by constituent devices in the growing system. In the present thesis, benzoic, caffeic, chlorogenic, ferulic, p-hydroxybenzoic, salicylic, vanillic acids, phenazine-1-carboxylic acid, pyoluteorin, pyrrolnitrin and 2,4-diacetyl phloroglucinol were studied with respect to occurrence in the nutrient solution of closed hydroponic growing systems, phytotoxic response, persistence and optimised formation. Phytotoxicity levels were studied on young tomato plants exposed to initial concentrations of benzoic, caffeic, chlorogenic, ferulic, p-hydroxybenzoic, salicylic, vanillic acids in the fresh solution. Effects were seen primarily on roots at 200 and 400 µM for most of the compounds. Supported liquid membrane technique was adopted to extract several plant and microbial metabolites in situ in the greenhouse. The method was then used to study the nutrient solution in tomato, cucumber and gerbera crops, which were grown in closed growing systems including artificial infection of root pathogenic fungi and/or selected disinfestation methods, namely slow sand filtration and UV-radiation. They were investigated at different developmental stages and compounds were determined in the range of 10-200 nM in the nutrient solutions. Benzoic acid was found throughout all crops, sampling occasions and treatments. Furthermore, p-hydroxybenzoic acid was determined, as well as occasionally 2,4-diacetyl phloroglucinol, especially in systems with slow sand filter treatment. In vitro, synthetical organic compounds added to effluent nutrient solution disappeared rapidly, mostly within two days. The occurrence of plant and microbial metabolites should rather be regarded as an asset. Optimising the living conditions for the resident microflora in the nutrient solution might favour biocontrol of root pathogens. Increased metabolite production of the control strain was observed. However, low levels of selected microbial metabolites were found after enrichment of the nutrient solution microflora. Preconditions for future studies are discussed.