Plant Physiol, December 1999, Vol. 121, pp. 1069-1079
Apoplastic pH and Fe3+ Reduction in Intact
Sunflower Leaves1
Harald U.
Kosegarten,
Bernd
Hoffmann, and
Konrad
Mengel*
Institut für Pflanzenernährung, Südanlage 6, Justus-Liebig Universität Giessen, D-35390 Giessen, Germany
(H.U.K., K.M.); and Nicht Invasive-Systeme, Silcherstrasse 72, D-73430
Aalen, Germany (B.H.)
It has been hypothesized that under
NO3
nutrition a high apoplastic pH in leaves
depresses Fe3+ reductase activity and thus the subsequent
Fe2+ transport across the plasmalemma, inducing Fe
chlorosis. The apoplastic pH in young green leaves of sunflower
(Helianthus annuus L.) was measured by fluorescence
ratio after xylem sap infiltration. It was shown that
NO3 nutrition significantly increased
apoplastic pH at distinct interveinal sites (pH 
6.3) and was
confined to about 10% of the whole interveinal leaf apoplast. These
apoplastic pH increases presumably derive from
NO3/proton cotransport and are supposed to be
related to growing cells of a young leaf; they were not found in the
case of sole NH4+ or
NH4NO3 nutrition. Complementary to pH
measurements, the formation of Fe2+-ferrozine from
Fe3+-citrate was monitored in the xylem apoplast of intact
leaves in the presence of buffers at different xylem apoplastic pH by means of image analysis. This analysis revealed that Fe3+
reduction increased with decreasing apoplastic pH, with the highest rates at around pH 5.0. In analogy to the monitoring of
Fe3+ reduction in the leaf xylem, we suggest that under
alkaline nutritional conditions at interveinal microsites of increased
apoplastic pH, Fe3+ reduction is depressed, inducing leaf
chlorosis. The apoplastic pH in the xylem vessels remained low in the
still-green veins of leaves with intercostal chlorosis.
1
This work was supported by the Deutsche Forschungsgemeinschaft.
*
Corresponding author; e-mail
konrad.mengel{at}ernaehrung.uni-giessen.de; fax 49-641-99-39169.
© 1999 American Society of Plant Physiologists
