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.
Key words
Apoplastic pH - ascorbate - dehydroascorbate - energized transport - plasma membrane
- proton motive force
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heber@botanik.uni-wuerzburg.de
Section Editor: U. Lüttge
