Characterisation of root amino acid exudation in white clover (Trifolium repens L.)

The mechanism of root amino acid exudation was studied in white clover (Trifolium repens L.) to explain the apparent selectivity of this process resulting in contrasted amino acid profiles between root tissues and root exudates. Asparagine is generally recovered in low proportions in root exudates but represent a major fraction of amino acids in root tissues, and the opposite is found for glycine. Amino acid profiles were studied in two potential sites of intense exudation, root tips and nodules, and a N-15 labelling method was used to compare influxes and effluxes of different amino acids. Metabolic inhibitors were used to test the assumption that amino acid exudation is a passive process. We observed that amino acid profiles were similar between whole roots, root tips and nodules. Influxes of asparagine and glycine were in the same range, but efflux of glycine largely exceeded efflux of asparagine. Metabolic inhibitors strongly reduced glycine influx and modified glycine efflux. It is concluded that the selectivity of amino acid exudation is not explained by the composition of putative intense sites of exudation but is explained by highly contrasted effluxes between amino acids. The effect of metabolic inhibitors shows that glycine efflux occurs not only as a simple passive leakage. These results suggest that amino acid exudation is a plant-controlled process.