The Kinetics of Phloem Loading of Valine in the Shoot of a Nodulated Legume (Lupinus albusL. cv. Ultra)

Abstract

Phloem loading of several amino acids (D- and L-Val, Arg, Asn, Asp, Leu) was studied in shoots of L. albus using a phloem bleeding technique on both intact plants and detached shoots fed via the transpiration stream. Val was singled out for intensive study due to the minimal amount of metabolism it underwent in the shoot For the amino acids studied, the relationship between xylem, phloem, and leaflet concentrations was determined by the interaction of rates of xylem supply, metabolism, and export. At elevated xylem fluid concentrations, low rates of loading of D-Val into the phloem and little metabolism in the tissues resulted in high levels in the leaflets. For other amino acids (Arg, Asp, Leu) rapid metabolism in the leaflets prevented a build-up in concentration in either phloem or leaflets. Asn was rapidly transferred to the phloem, thus high levels in the xylem lead to high concentrations in the phloem without greatly affecting leaflet concentrations. L-Val responded in a manner intermediate between Asn and D-Val. A detailed study of L-Val showed it to be loaded into the phloem against a concentration gradient in both stem and leaflets. Some of this Val originated from the transpiration stream at both locations but in the leaflets as much as 64% of the Val originated from other sources, e.g. recent photosynthesis. L-Val supplied to the phloem in the stem was derived from a large storage pool and did not come directly from the xylem fluid. As a consequence the rate of stem loading was independent of short-term fluctuations in the xylem fluid Val concentration. L-Val entering the leaflets in the xylem initially bypassed the large storage pool and was loaded directly into the phloem. However, after 350 min the pools had reached an equilibrium and rate of phloem loading was dependent on total leaflet concentration.