SUGAR AND AMINO-ACID-COMPOSITION OF PHLOEM SAP OF MEDICAGO-SATIVA - A COMPARATIVE-STUDY OF 2 COLLECTING METHODS

The sugar and amino acid composition of the phloem sap of alfalfa (Medicago sativa) was studied. Sap was collected by stylectomy (using radio frequency microcautery) or by facilitated exudation with ethylene diamine tetraacetic acid (EDTA). Stylectomy parameters (section and exudation percentages) demonstrated the reliability of the stylectomy apparatus with the biological system Medicago sativa/Acyrthosiphon pisum. In the phloem sap of alfalfa obtained by stylectomy, the only sugar present was sucrose (its concentration was about 390 mM). Asparagine which accounted for 70% (w/w) of total nitrogenous compounds was the major form of transport of reduced nitrogen in the phloem. Noticeably, the amino acid content was as high as the sugar content. Comparing the data from stylectomy with those obtained from exudation experiments showed that the hexoses and gamma-amino butyric acid recovered in the sap collected after exudation are artefacts resulting from the cutting of the petioles. Furthermore, the amide/dicarboxylic acid ratio is significantly lower in the exudation sap than in the stylectomy sap. Stylectomy is an excellent technique for fine biochemical analysis of phloem sap composition but exudation with EDTA is convenient for rough analysis or to test the phloem mobility of xenobiotics.