Amino acids exuded from axenic roots of lettuce and white lupin seedlings exposed to different cadmium concentrations

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Abstract
The objective of this study was to evaluate if amino acids in roots and/or in root exudates play a role in cadmium (Cd) stress. Lettuce (Lactuca sativa L. cv. Reine de Mai) and white lupin (Lupinus albus L. cv. lublanc) were grown for 19 to 21 days with axenic roots in a hydroponic system. After treatment with various concentrations of Cd (0, 0.01, 0.1, 1, 10, and 100 μM Cd) per nine days, roots and root exudates were collected. The stress did not result in significant dry weight (DW) differences between Cd‐treated and control plants, but Cd induced decreases in relative water content (RWC) and water potential (Pm). Amino acid levels and carbon (14C) incorporation into amino acids increased at low Cd concentrations in roots. However, 100 μM Cd induced a decrease of amino acid levels and an equally significant reduction of 14C incorporation, suggesting a decreased plant metabolism. Moreover, a higher Cd concentration induced increased levels of specific amino acids, for instance asparagine and lysine in lettuce and asparagine and hydroxylysine in lupin roots. Amino acids in root exudates corresponded less than 1% of the amounts found in root cells suggesting that amino acids could not be the major Cd chelators. Amino acid accumulation in root exudates differed than that found in roots except for asparagine. In conclusion, Cd induces in the root and root exudates increased levels of specific amino acids, such as Asn, Lys and HLys similarly to other environmental stresses. Although the amino acids could not participate in Cd chelation, lysine and its derivatives, such as hydroxylysine, could be used as stress markers for Cd in higher plants.