Plant Biol (Stuttg) 2003; 5(2): 151-158
DOI: 10.1055/s-2003-40721
Original Paper

Georg Thieme Verlag Stuttgart · New York

Energized Uptake of Ascorbate and Dehydroascorbate From the Apoplast of Intact Leaves in Relation to Apoplastic Steady State Concentrations of Ascorbate

U. Heber 1 , N. G. Bukhov 2 , Ch. Wiese 1 , R. Hedrich 1
  • 1Julius-von-Sachs-Institut für Biowissenschaften, Universität Würzburg, Würzburg, Germany
  • 2Timiriasev Institute of Plant Physiology, Russian Academy of Sciences, Moscow, Russia
Further Information

Publication History

Publication Date:
21 July 2003 (online)

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Abstract

Transport of ascorbate (AA) and dehydroascorbate (DHA) through the petiole into detached leaves of Lepidium sativum and other plant species via the transpiration stream, and energized uptake into leaf tissue, were measured indirectly by recording changes in membrane potential and apoplastic pH simultaneously with substrate-stimulated respiration and transpiratory water loss. When 25 mM AA or DHA was fed to the leaves, steady state respiration at 25 °C was transiently increased by more than 50 % with AA and 70 % with DHA. Stimulation of respiration was accompanied by a transient breakdown of membrane potential followed by alkalinization of the leaf apoplast suggesting energized uptake at the expense of the transmembrane proton motive force. The average CO2/AA ratio calculated from stimulated respiration during ascorbate uptake was 0.76 ± 0.26 (n = 17). The corresponding ratio for DHA was 1.38 ± 0.28 (n = 11). Far lower CO2/substrate ratios were observed when NaCl or KCl were fed to leaves. The differences indicate either partial metabolism of AA and DHA in addition to energized transport, or less likely, higher energy requirement for transport of AA and DHA than for the inorganic salts. Maximum rates of energized AA transport into leaf tissue (deduced from maxima of extra respiration and calculated on the basis of CO2/AA = 0.76) were close to 650 nmol m-2 leaf area s-1, i.e. far higher than most previously reported rates of transport. When the apoplastic concentration of AA was decreased below steady state levels during infiltration/centrifugation experiments, AA was released from leaf cells into the apoplast. This suggests that AA oxidation to DHA in the apoplast (as occurs during extracellular ozone detoxification) triggers energized transport of the DHA into the symplast and simultaneously AA release from the symplast into the apoplast, perhaps together with protons in a reversal of the energized uptake process.

References

U. Heber

Julius-von-Sachs-Institut für Biowissenschaften
Universität Würzburg

97084 Würzburg

Germany

Email: heber@botanik.uni-wuerzburg.de

Section Editor: U. Lüttge