Photosynthetic path of carbon dioxide in spinach and corn leaves

• Published in: The Journal of biological chemistry Vol. 249; no. 11; pp. 3436 - 3441
• Main Authors: Laber, L.J, Latzko, E, Gibbs, M
• Format: Journal Article
• Language: English
• Published: United States American Society for Biochemistry and Molecular Biology 10-06-1974
• Subjects: Carbon Dioxide - metabolism; Carbon Radioisotopes; Darkness; Glyceric Acids - metabolism; horticultural crops; Ketoses - metabolism; Light; Nitrogen; Organophosphorus Compounds - metabolism; Oxygen; Partial Pressure; Pentosephosphates - metabolism; Phosphoenolpyruvate - metabolism; Phosphoenolpyruvate Carboxykinase (GTP) - metabolism; Photosynthesis; plant biochemistry; plant physiology; Plants - enzymology; Plants - metabolism; Species Specificity; Time Factors; Zea mays - enzymology; Zea mays - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)42591-X

The primary route of CO 2 incorporation in C-4 plants (corn) has been formulated to be β-carboxylation catalyzed by phosphoenolpyruvate carboxylase,
while in C-3 plants (spinach) CO 2 enters the carboxyl carbon of glycerate 3-phosphate after reacting with ribulose 1,5-diphosphate. Detached leaf tissues of
spinach and corn were compared with respect to their fixation of CO 2 in the dark or in the light subsequent to illumination in the absence of CO 2 in order to accumulate the primary compound for carboxylation.
Prior illumination of corn and spinach leaves in an atmosphere of N 2 containing 1.5% O 2 , enhanced dark 14 CO 2 fixation 15- to 25-fold over that of unilluminated leaves. In corn the primary products (80%) of enhanced dark fixation were
malate and aspartate. Less than 5% of the incorporated 14 CO 2 appeared in glycerate 3-phosphate and other intermediates of the reductive pentose phosphate cycle. In spinach, roughly one-half
of the isotope was found in glycerate 3-phosphate and sugar phosphates with about 25% in alanine.
The levels of ribulose 1,5-diphosphate, glycerate 3-phosphate, and phosphoenolpyruvate were determined during preillumination
under an atmosphere containing N 2 and 1.5% O 2 , after pulses of CO 2 either in a light or in a dark period. In both tissues ribulose 1,5-diphosphate increased linearly during preillumination
and decreased rapidly following the introduction of CO 2 . Glycerate 3-phosphate was little affected during preillumination but increased rapidly upon addition of CO 2 . In both species the level of phosphoenolpyruvate was essentially unchanged during preillumination but increased sharply
in corn leaves with introduction of CO 2 . CO 2 pulses had no effect on the level of phosphoenolpyruvate in spinach leaves.
It was concluded that ribulose 1,5-diphosphate can serve as a primary acceptor of CO 2 in detached leaves of C-4 (corn) and C-3 plants (spinach).