We investigated the effects of glucose and root exudates in combination with different nitrogen (N) forms on the spore germination rate and hyphal growth of the arbuscular mycorrhizal (AM) fungus Rhizophagus intraradices. Spores were cultured for 20 days at 26 °C in agar medium containing either glucose or root exudates at different concentrations. Nitrogen was supplied in the form of 4 mM NaNO 3 , 4 mM NH 4 Cl, 4 mM NH 4 NO 3 or 2 mM urea. Three different controls without added N were used:

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... onized water, glucose, and root exudates. After 20 days, Glucose concentrations of 10, 25, 40 and 50 g L -1 inhibited the spore germination; however, when the concentration of glucose was 5 g L -1 , the mean hyphal length was longer than that in the control. Glucose plus different forms of N sources also has no positive effect on the germination as well as hyphal growth. The addition of root exudates significantly improved both the spore germination rate and hyphal lengths in all four media (root exudates +4 mmol NaNO 3 , root exudates +4 mmol NH 4 Cl, root exudates +4 mmol NH 4 NO 3 and root exudates +2 mmol urea). After incubation for 20 days, the highest spore germination rates (80.33%) were detected in the root exudate + urea medium, while that in the root exudate + NaNO 3 medium was lower than in the control. In all these four media, hyphal growth was initially rapid and slowed in the later stages of incubation. Finally, the greatest hyphal length (15. 15 mm spores -1 was observed in the root exudate + NH 4 NO 3 medium, while the lowest was observed in the exudate + NaNO 3 medium.