Juglone reduces growth, nitrogenase activity, and root respiration of actinorhizal black alder seedlings

European black alder trees [Alnus glutinosa (L.) Gaertn.] fix nitrogen with nodular symbionts and are interplanted with valuable black walnut trees (Juglans nigra L.) to increase soil nitrogen fertility. However, on some soils interplanted alder can be killed by black walnut's allelochemical juglone. In order to better understand the effects of juglone directly on the growth, nitrogen fixation, and root respiration of black alder, we grew nodulated alders hydroponically in a nitrogen-free nutrient solution at juglone levels of 2 × 10(-6), 2 × 10(-5), and 0 molar (M). Results indicate that nitrogenase activity (acetylene reduction) of alders growing in 2 × 10(-3) M juglone was reduced relative to alders without added juglone after one day, and in 2 × 10(-6) M juglone after five days. Root respiration (CO2 evolution) and the relative increase of plant fresh weight were reduced in the 2 × 10(-5) M juglone treatment. In a related experiment, black alder germinants were grown in Flanagan silt loam soil dosed with 10(-3), 10(-4), and 0 M juglone. The inhibitory effects of 10(-3) M juglone on radicle elongation ceased 22-37 days after juglone treatments were started, suggesting that this soil can readily detoxify juglone.