ARE PHLOEM AMINO, ACIDS INVOLVED IN THE SHOOT TO ROOT CONTROL OF NO3-UPTAKE IN RICINUS-COMMUNIS PLANTS

The putative role of phloem amino acids as negative feedback signals f or root NO3- uptake was investigated in Ricinus communis L, The NO3--g rown plants were subjected to N-deficiency due either to complete N-de privation, or to localized N-deprivation on one side of a split-root s ystem. in comparison with controls, complete N-deprivation resulted in a transient increase in (NO3-)-N-15 influx, and in profound changes i n downward phloem transport of amino acids. Total amino acid concentra tion in the phloem sap decreased by 40%, but responses markedly differ ed between the individual amino acids. Concentrations of Gln and Ser w ere rapidly lowered by 50%, while those of Val, Phe, Leu, and lie disp layed a marked increase. Localized N-deprivation on one side of the sp lit root system also resulted in the up-regulation of (NO3-)-N-15 infl ux in the roots still supplied with NO3-. However, the amino acid comp osition of the phloem sap directed to these roots was not modified by the treatment, and remained similar to that in N-sufficient control pl ants. Only amino acid transport to the N-deprived roots was affected a s observed in response to complete N-deprivation. The results from spl it-root plants indicate that the response of root NO3- influx to N-def iciency is controlled by shoot-borne regulatory signals, and provide a case study where these signals are not related to a qualitative chang e or a significant decrease in downward phloem transport of amino acid s.