Acquisition and Assimilation of Nitrogen as Peptide-Bound and D-Enantiomers of Amino Acids by Wheat

Nitrogen is a key regulator of primary productivity in many terrestrial ecosystems. Historically, only inorganic N (NH 4 + and NO 3 - ) and L-amino acids have been considered to be important to the N nutrition of terrestrial plants. However, amino acids are also present in soil as small peptides and in D-enantiomeric form. We compared the uptake and assimilation of N as free amino acid and short homopeptide in both L- and D-enantiomeric forms. Sterile roots of wheat ( Triticum aestivum L.) plants were exposed to solutions containing either 14 C-labelled L-alanine, D-alanine, L-trialanine or D-trialanine at a concentration likely to be found in soil solution (10 µM). Over 5 h, plants took up L-alanine, D-alanine and L-trialanine at rates of 0.9±0.3, 0.3±0.06 and 0.3±0.04 µmol g −1 root DW h −1 , respectively. The rate of N uptake as L-trialanine was the same as that as L-alanine. Plants lost ca. 60% of amino acid C taken up in respiration, regardless of the enantiomeric form, but more ( ca. 80%) of the L-trialanine C than amino acid C was respired. When supplied in solutions of mixed N form, N uptake as D-alanine was ca. 5-fold faster than as NO 3 - , but slower than as L-alanine, L-trialanine and NH 4 + . Plants showed a limited capacity to take up D-trialanine (0.04±0.03 µmol g −1 root DW h −1 ), but did not appear to be able to metabolise it. We conclude that wheat is able to utilise L-peptide and D-amino acid N at rates comparable to those of N forms of acknowledged importance, namely L-amino acids and inorganic N. This is true even when solutes are supplied at realistic soil concentrations and when other forms of N are available. We suggest that it may be necessary to reconsider which forms of soil N are important in the terrestrial N cycle.