SEASONAL-VARIATION OF AMINO-ACIDS IN THE XYLEM SAP OF POPULUS X CANADENSIS AND ITS RELATION TO PROTEIN BODY MOBILIZATION

The seasonal changes in the pattern of 21 amino acids occurring in the xylem vessels of Populus twigs have been studied in connection to the mobilization of protein bodies in ray parenchyma cells at the electron microscopic level. Hydrolysis of protein bodies in spring and movement of amino acids into vessels are found to be closely linked. Comprising more than 75% of total amino acid content, glutamine (Gln) is by far the dominant N-constituent of the sap. Gln reaches up to 11 mumol ml-1 in the spring sap while other amino acids only show 1/20 to 1/1 00 of this amount. From the measured Gln accumulation rates in the vessels in nature and in the vessels of isolated shoots, a minimum flux rate for Gln of 5.6 pmol cm-2 min-1 is calculated for the ray contact cell/vessel interface. Furthermore, because Gln constitutes 75% of the amino acid content of the sap but only 1.3% of the amino acids in the 32 kDa storage protein of the ray cells in the wood (Clausen and Apel 1991), it becomes evident that most amino acids originating from protein body mobilization do not enter the vessels but are used for Gln synthesis preceding Gln release into the vessels.