title: Inactivation of the photosynthetic carbon reduction cycle in isolated mesophyll protoplasts subjected to freezing stress.
abstract: Isolated mesophyll protoplasts from Valerianella locusta L. were subjected to freeze-thaw cycles. Subsequently, steady-state pool sizes of (14)C-labeled intermediates of the photosynthetic carbon reduction cycle were determined by high performance liquid chromatography. Protoplasts in which CO2 fixation was inhibited by preceding freezing stress, showed a strong increase in the proportion of fructose-1,6-bisphosphate, sedoheptulose-1,7-bisphosphate and triose phosphates. These results indicate an inhibition of the activities of stromal fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase. Furthermore, freezing stress caused a slight increase in the proportion of labeled ribulose-1,5-bisphosphate, which may be based on an inhibition or ribulose bisphosphate carboxylase activity. It was shown earlier (Rumich-Bayer and Krause 1986) that freezing-thawing readily affects photosynthetic CO2 assimilation independently of thylakoid inactivation. The present results are interpreted in terms of an inhibition of the light-activation system of the photosynthetic carbon reduction cycle, caused by freezing stress.
authors.affiliation: Botanisches Institut der Universität Düsseldorf, Universitätsstrasse 1, D-4000, Düsseldorf 1, Federal Republic of Germany.
authors.forename: S
authors.initials: S
authors.lastname: Rumich-Bayer
authors.forename: C
authors.initials: C
authors.lastname: Giersch
authors.forename: G H
authors.initials: GH
authors.lastname: Krause