INHIBITION OF NO3- UPTAKE BY VARIOUS PHLOEM-TRANSLOCATED AMINO-ACIDS IN SOYBEAN SEEDLINGS

We have examined the effect of different amino acids on NO3- uptake in 5-d-old soybean (Glycine max L. Merr. cv. Kingsoy) seedlings. Amino acids were supplied to the root either by addition to the nutrient solution, or by immersion of the tip-cut cotyledons in a concentrated solution. NO3- uptake was measured by its depletion in amino acid-free root medium. Provision of amino acid to the roots resulted in an inhibition of NO3- uptake, mainly with Asp, Glu, Asn, Arg, Ala, beta-Ala, less with Ser and Gln, and even less with other amino acids. Using Arg and beta-Ala, we have shown that the supplied amino acid accumulated in the root tissues. Immersing the cotyledons in a concentrated solution of amino acid resulted both in an enrichment of the phloem exudate and in an inhibition of NO3- uptake. With this protocol, the most important inhibition was obtained with Arg, Ala, beta-Ala, Asn, Gln, Glu, Met, and Asp. Moreover, the level of inhibition was lower than that obtained with the root-feeding. The significance of such an inhibitory effect of phloem-translocated amino acids on NO3- uptake as a regulatory signal for matching NO3- uptake rates to N demand in older plants is discussed.