Effects of walnut (Juglans regia L.) root exudates on germination, seedling growth and enzymatic activities of turnip (Brassica rapa L.)

To determine the allelopathic effects of root exudates of walnut trees, the root exudates of 2, 3, and 4-years-old walnut (Juglans regia L.) trees were extracted with the neutral, acidic (pH 3.0), and alkaline (pH 8.0) components of ethyl acetate extract. The bioactivity of these 3-components (neutral, acidic and alkaline) were separately investigated on the seed germination, seedling growth, the activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and the content of malondialdehyde (MDA) of turnip (Brassica rapa L.). The 3-components of ethyl acetate extract of root exudates of walnut trees inhibited the seed germination and seedling growth of turnip as compared to control. Of all the enzymes assayed (SOD, POD, CAT and MDA), the SOD and the POD activity played a cooperative effect in clearing the reactive oxygen radicals. In 3-treatments (neutral, acidic and alkaline) of root exudates of 4-years-old walnut trees, the activities of SOD, POD and CAT enzymes in turnip were higher than the 3-treatments of root exudates of 2 and 3-years-old walnut trees, whereas the acitivity of MDA was lower. Thus in-vitro allelopathic effects of walnut root exudates were more pronounced with the increasing age of walnut trees. The in-vitro inhibitory/stimulatory effects of root exudates of walnut trees varied with the pH of ethyl acetate extract and with the age of walnut trees.