Allelopathy in black walnut (Juglans nigraL.) alley cropping. II. Effects of juglone on hydroponically grown corn (Zea maysL.) and soybean (Glycine maxL. Merr.) growth and physiology

We conducted an experiment to investigate the effects of juglone (5-hydroxy-1, 4-napthoquinone) on the growth and physiology of hydroponically grown corn (Zea mays L.) and soybean (Glycine max L. Merr.) seedlings. Three different concentrations of juglone (10^-6 M, 10^-5 M, and 10^-4 M) along with a control were applied. Within 3 days, juglone exhibited significant inhibitory effects on all measured variables including shoot and root relative growth rates (RGR_s and RGR_r), leaf photosynthesis (P_net), transpiration (E), stomatal conductance (g_s), and leaf and root respiration. In general, soybean was found to be more sensitive to juglone than corn. RGR_r was the most inhibited variable for both species, and reductions of 86.5 and 99% were observed in corn and soybean, respectively, with 10_-4 M juglone concentrations. Among the physiological variables measured, P_net showed the greatest impact of toxicity though the other physiological parameters were also impacted. We conclude that both corn and soybean are sensitive to juglone and observed growth reductions in corn and soybean in black walnut alley cropping may partly be due to juglone phytotoxicity. Determination of actual phytotoxicity will require quantification of soil solution juglone levels, particularly in areas where soil solid-phase levels are high in close proximity to trees.