This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by van Bel, A. J. E. Articles by Borstlap, A. C. Search for Related Content PubMed PubMed Citation Articles by van Bel, A. J. E. Articles by Borstlap, A. C. Agricola Articles by van Bel, A. J. E. Articles by Borstlap, A. C.

Articles

Kinetics of L-Alanine Escape from Xylem Vessels

¹ Botanical Laboratory, State University of Utrecht, Lange Nieuwstraat 106, Utrecht, The Netherlands

Labeled (³H or ¹⁴C) L-alanine was perfused through the xylem vessels of isolated tomato internodes (Lycopersicon esculentum cv. Moneymaker) at various concentrations (10^–6 molar to 10^–2 molar). At each concentration the escape of L-alanine from the xylem vessels was apparently a first order process, which is in agreement with Horwitz' (1958, Plant Physiology 33:81-93) model for irreversible escape from the xylem vessels. The escape constant (K) decreased at higher concentrations of L-alanine, which implies that Horwitz' model is inappropriate to describe the kinetics of L-alanine escape, and that the escape at least partly is a saturable process. To obtain data that relate the concentration of L-alanine in the xylem vessels and the escape rate of the amino acid, average escape rates per internode were measured and the corresponding concentrations were calculated from the integrated form of the Michaelis-Menten equation.

As the concentration dependence of the escape rate was biphasic, three possible mechanisms were considered, escape being caused by: (a) saturable amino acid uptake of cells around the xylem vessels and diffusion into the free space; (b) saturable uptake of the cells around the xylem vessels, but at higher amino acid concentrations in the xylem vessels the number of cells, that participate in the uptake, increases; (c) two, simultaneously operating, saturable uptake systems in the cells around the xylem vessels.