WATER DEFICIT-INDUCED CHANGES IN CONCENTRATIONS IN PROLINE AND SOME OTHER AMINO-ACIDS IN THE PHLOEM SAP OF ALFALFA

Changes in amino acid composition of alfalfa (Medicago sativa L.) phlo em sap were studied in response to a water deficit. Sap was collected by stylectomy. As the leaf water potential (Psi) decreased from -0.4 t o -2.0 MPa, there was a significant increase of the total amino acid c oncentration, due to that of some amino acids: proline, valine, isoleu cine, leucine, glutamic acid, aspartic acid, and threonine. Asparagine concentration, which is the main amino acid assayed in the phloem sap of alfalfa (it accounts for 70% of the total content), did not vary w ith the plant water status. The other amino acid concentrations remain ed stable as Psi varied; in particular, gamma-amino butyric acid conce ntration remained unchanged, whereas it varied in response to wounding . The more striking change in the sieve tubes was the accumulation of proline, which was observed below a Psi threshold value of about -0.9 MPa (concentration x60 for a decrease of Psi from -0.9 to -2.0 MPa). T he role of such changes in phloem sap amino acid concentration in osmo tic adjustment of growing tissues is discussed.