Quantitative and qualitative comparison of root exudates of mycorrhizal and nonmycorrhizal plant species

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Abstract
At 6 weeks after seed germination, the rate of leakage of carbohydrates and amino acids from roots was significantly higher in three plant species (Sorghum vulgare Pers., Lycopersicon esculentum Mill., and Citrus aurantium L.) that normally form vesicular–arbuscular mycorrhizae (VAM) than in three species (Chenopodium quinona Willd., Phytolacca americana L., and Saponaria vaccaria L.) that do not form VAM. There were no sugars, amino acids, or carboxylic acids that were consistently detected in root exudates of VAM-forming species that were also consistently absent from the exudates of non-VAM species. Although rates of root exudation over long time periods were lower in non-VAM species, at early stages of seedling establishment, the rate of exudation was greater in roots of Chenopodium quinona than in roots of Citrus aurantium. These results suggest that the overall rate of exudation may be an important factor in maintaining VAM in most plant species, but no specific compound could be identified as promoting VAM formation.