Patterns of short-term amino acid accumulation and loss in the root-zone of liquid-cultured forage rape (Brassica napus L.)

Amino acid release from roots of sterile and non-sterile, solution-grown, 7-, 21- and 60-days-old forage rape plants (Brassica napus L.), was measured over periods of up to 6 hours. With sterile plants, release of amino acids into a fixed volume of collection medium (6, 12, 70 mL) was concentration-limited, giving rise to similar convex accumulation profiles for individual acids. In contrast, amino acid accumulation in continuously circulating collection medium was not concentration limited, giving a linear accumulation pattern. The compositions of accumulating amino acids, which were similar to those measured in root extracts, did not change significantly. However, the proportions of ALA, GABA, GLU and ILE in both root extracts and root-derived amino acids increased as plants aged. Older plants released more amino acids per plant, while younger plants released more amino acids g^-1 root DW. Using non-sterile plants, the patterns of change in amino acid concentration and composition in the collection medium were completely different from those determined with sterile plants. In general, with 7-days-old plants, and 60-days-old plants that had recently become non-sterile, an initial rise in the concentration of all acids was followed by a fall to low levels. The loss of amino acids was apparently due to microbial consumption. Individual amino acids attained maximum concentration at different times during the collection process. This is attributed mainly to concentration-dependent differential assimilation of amino acids, since those with the highest initial concentrations, the major components of the mixtures released from roots, declined the earliest. When calculated rates of amino acid release from roots (R_r) and microbial consumption of amino acids (R_c) were compared (for 7-days-old plants), the highest ratios of R_c/R_r were found for ASN, ARG, GLU, GLN, and LYS. This suggests a degree of selectivity for glutamate and nitrogen-rich acids on the part of the consuming micro-organisms. With 21-days old plants and 60-days old plants grown entirely under non-sterile conditions, fluctuations in amino acid concentration were similar for all acids.