Kinetics of l-Alanine Escape from Xylem Vessels

Abstract

Labeled (3H or "C) 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 ceUs, that participate in the uptake, increases; (c) two, simultaneously operating, saturable uptake systems in the cells around the xylem vessels.In the root system of many higher plants inorganic nitrogen taken up from the soil is converted into organic nitrogen, part of which is translocated to the shoot via the xylem vessels. The main organic nitrogen compounds in the xylem sap are amino acids and amides (1,13,15). In tomato, the concentration of amino acids in the xylem bleeding sap is between 1 and 5 mm (8,20).Evidence has been obtained that under our experimental conditions, the free space of the xylem translocation path extends into the phloem area (18), so that direct transfer of amino acids from xylem to phloem may occur (cf. 9). The escape of amino acids from the transpiration stream is therefore, an important process in the nitrogen supply of stem and leaves. Investigations on this process in isolated internodes of tomato showed that in perfusion experiments the amount of amino acid that escapes from the xylem vessels is quantitatively absorbed by the surrounding living cells (16,19,21).The kinetic data of the escape process have been interpreted, so far, by use of the model of Horwitz (5,16,17), in which the escape is regarded as a first order process. Inasmuch as amino acids are selectively absorbed from xylem vessels (16,19,21), it may be expected that the escape will show saturation kinetics. Such kinetic data have been also found for the uptake of amino acids by other plant tissues (2,7,11,12,22). The experiments described in this paper were carried out to obtain more information on the escape kinetics of amino acids from xylem vessels. MATERIALS AND METHODS Cultivation of Plants and PerfusionTechnique. The cult...

Distribution and Metabolism of Xylem-Borne Ureido and Amino Compounds in Developing Soybean Shoots

“…Here, as suggested earlier ( 17), the major xylem-borne nitrogenous solutes, ALA and ALA, were readily metabolized (Table III), and the extent of direct xylem-to-phloem transfer remained uncertain. In cowpea, a species that also exports principally ureides from root nodules, investigations using aphids (3) and cryopunctured fruits (16) Previous research has shown that the concentration of xylem-borne amino acids is an important factor in the uptake and distribution pattern in tomato (22) and cottonwood (23) stems. Therefore, we supplied ureido and amino compounds at high specific activity against the background of the solutes that are normally delivered from nodulated roots to shoots in the xylem stream.…”

Light-stimulated biphasic amino acid uptake by xylem parenchyma cells

“…The present results indicate that the biphasic kinetics in the escape of alanine from the xylem vessels [ 9] can be attributed to the action of the xylem parenchyma cells. It has been mentioned [9] that the model of Ehwald et al [8] can explain the biphasic character of the escape from the vessels. This concept, however, does not give a solution for biphasic uptake by cell suspensions [3,5].…”

“…In tomato internodes, the alanine escape from the xylem vessels appeared to be biphasic [9] and the escape of amino acids from the xylem vessels is accompanied with proton transport [ 10,11]. The escape might reflect the uptake activity of the phloem tissues, since it has been demonstrated that amino acids can rapidly traverse to the phloem [12].…”

Potassium co-transport and antiport during the uptake of sucrose and glutamic acid from the xylem vessels

“…The leakage pattern of glutamic acid corresponds with those earlier found for other amino acids: after 10--20 rain of [14C]amino acid perfusion a steady leakage level establishes, during which a constant fraction of the labelled material escapes from the xylem vessels [10,11]. Most of the escaped radioactivity is actively taken up by the cells around the xylem vessels [ 12,13]. If co-transport is taking place, steady uptake of glutamic acid must be accompanied with a steady decrease of the amount of protons and a steady increase of the K + content in the perfusate.…”