Different Mass Transfer Rates of Labeled Sugars and Tritiated Water in Xylem Vessels and Their Dependency on Metabolism

Bel, Aart J. E. van

doi:10.1104/pp.57.6.911

Cited by 29 publications

(9 citation statements)

References 24 publications

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“…The cultivation of the plants (Lycopersicon esculentum cv. Moneymaker) and the perfusion technique have been described before (17). Briefly, labeled amino acids were perfused through excised tomato internodes and the radioactivity in the perfusate was counted by liquid scintillation spectrometry.…”

Section: Methodsmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

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Kinetics of l-Alanine Escape from Xylem Vessels

Bel

Mostert²,

Borstlap³

1979

Plant Physiol.

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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...

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Kinetics of l-Alanine Escape from Xylem Vessels

Bel

Mostert²,

Borstlap³

1979

Plant Physiol.

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“…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].…”

Section: Resultsmentioning

confidence: 78%

“…Growth and peffusion methods have been described elsewhere [ 10]. ~4C~LabeUed solutions (5 raM) of glutamic acid, glutamine or sucrose (in some experiments diluted in buffers) were perfused through excised internodes.…”

Section: Methodsmentioning

confidence: 99%

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

Bel

Erven

1979

Plant Science Letters

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SUMMARYPerfusion experiments with excised internodes of tomato (Lycopersicon esculentum cv Moneymaker) showed that the uptake of glutamic acid and sucrose from the xylem vessels is accompanied with coupled proton cotransport and potassium antiport at low pH (<~5.5). At high pH (>5.5) both proton and potassium co-transport accompany the uptake. The results fit into the proton pump concept. INTRODUC~ONRecently we presented a model wherein protons and potassium ions (the latter only at high pH) are co-transported with carrier-substrate complexes across the membranes of tomato internode cells [1]. It may be summarised as follows:.At low pH (~5.5), protons are pumped out against an electrochemical gradient energised by the conversion of ATP into ADP by membrane-bound ATPase. The high proton-motive force (A pH + AE is high) drives protons, coupled to the carrier-substrate complex, through the membrane into the cells. After release of protons and substrate molecules, the carriers move back to the outer part of the membrane. The K ÷ ions leave the cells passively in order to reach their Nemst potential.At high pH (~5.5), the K ÷ ions tend to enter the cells passively, as the membrane potential is more negative than the Nernst potential of potassium ions. The proton-motive force which is smaller than at low pH (as A pH is smaller) drives the protons together with the carrier-substrate complex, through the membrane. The K ÷ ions compete with the protons for the binding sites of the carrier-substrate complex.Proton and potassium fluxes can be read off from changes in proton and

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