The carboxylate composition of rhizosheath and root exudates from twelve species of grassland and crop legumes with special reference to the occurrence of citramalate

Aims: Low-molecular-weight organic anions (carboxylates) influence rhizosphere processes and may enhance plant phosphorus acquisition. We examined the root exudate profile of a range of pasture and grain legumes and focused on the little-investigated carboxylate, citramalate. Methods: Twelve species of pasture legumes and herbs, including four Lotus spp. and two crop legumes, Cicer arietinum and Lupinus albus, were grown in a glasshouse for six weeks. The composition and amounts of carboxylates were measured in rhizosheath soil as well as in root exudates from roots washed free of their rhizosheath. Results: Citrate and malate were found in the rhizosheath of all species. However, citramalate was present in the rhizosheath of only Lotus species (10–82% of total exuded carboxylates). Cicer arietinum had the largest amount of carboxylates in its rhizosheath and fastest rate of carboxylate exudation into the trap solution. Conclusions: Citrate and malate were found in the rhizosheath of all species in this study, but citramalate was only found in the root exudates and rhizosheath of Lotus spp. Further investigation into the role of citramalate in Lotus spp. is merited.