Allelopathic effects of walnut (Juglans regia L.) rhizospheric soil extracts on germination and seedling growth of turnip (Brassica rapa L.)

The allelopathic effects of walnut rhizosphere soil were investigated by extracting the rhizosphere and adjacent soil beneath walnut trees using 20% methanol, methanol, ethyl acetate and hexane. The effects of extracts were investigated to determine their effects on turnip (Brassica rapa L.) seed germination and seedling growth, the activity levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and the malondialdehyde (MDA) content. All extracts inhibited the seed germination, seedling length and root length of turnip than control (soil adjacent to walnut trees). Of the enzymes assayed (SOD, POD, CAT and MDA), The ethyl acetate extract had most inhibitory effects on the activity of POD and MDA, while the methanol extracts were most inhibitory to MDA than control. This indicated that the three enzymes had cooperative effects in elimination of reactive oxygen radicals. We concluded that some compounds present in the rhizosphere soil of walnut trees (from the root exudates) had allelopathic effects on turnip.