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Pathway of Photosynthetic Malate Formation in Vitis vinifera, a C₃ Plant ¹

Institute of Plant Biology, University of Zürich, Zollikerstrasse 107, CH-8008 Zürich, Switzerland

The time course of intramolecular isotope distribution in phosphoglyceric acid and serine was determined after exposure of grape leaf discs (Vitis vinifera L.) to ¹⁴CO₂ (1000 microliters per liter) for variable metabolic periods, and the labeling patterns were compared with the respective isotope distribution in the C_1-3 fragment of malic acid. The results clearly support the classical concept of a close precursor-product relationship between photosynthetic phosphoglycerate and malic acid. Under the assimilatory conditions used in this study, there was no indication of an immediate carbon transfer from serine to malate as has been suggested for C₃ plants (Kent et al. 1974 Plant Physiol 53: 491-495) because of a coincident labeling of these compounds in Vicia faba. According to our data, previous evidence in favor of this hypothetical pathway is based largely on an unusual ¹⁴C distribution in serine, due to an extreme suppression of photorespiration, as well as on arbitrary comparisons between compounds of divergent kinetic characteristics and consequently different degrees of metabolic label randomization.