Abstract
Abstract The primary route of CO2 incorporation in C-4 plants (corn) has been formulated to be β-carboxylation catalyzed by phosphoenolpyruvate carboxylase, while in C-3 plants (spinach) CO2 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 CO2 in the dark or in the light subsequent to illumination in the absence of CO2 in order to accumulate the primary compound for carboxylation. Prior illumination of corn and spinach leaves in an atmosphere of N2 containing 1.5% O2, enhanced dark 14CO2 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 14CO2 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 N2 and 1.5% O2, after pulses of CO2 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 CO2. Glycerate 3-phosphate was little affected during preillumination but increased rapidly upon addition of CO2. In both species the level of phosphoenolpyruvate was essentially unchanged during preillumination but increased sharply in corn leaves with introduction of CO2. CO2 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 CO2 in detached leaves of C-4 (corn) and C-3 plants (spinach).
Highlights
SUMMARYThe primary route of COn incorporation in C-4 plants (corn) has been formulated to be fi-carboxylation catalyzed by phosphoenolpyruvate carboxylase, while in C-3 plants (spinach)
Species with the reductive pentose phosphate cycle or Calvin cycle incorporate atmospheric CO* directly into glycerate-3-P in the reaction catalyzed by ribulose-1,5-Pz carboxylase [1]
In contrast to corn leaves, glycerate-3-P and sugar phosphates accounted for the bulk of the ‘*CO2 assimilated by spinach tissue either as a result of photosynthesis or of dark fixation enhanced by prior illumination
Summary
The primary route of COn incorporation in C-4 plants (corn) has been formulated to be fi-carboxylation catalyzed by phosphoenolpyruvate carboxylase, while in C-3 plants (spinach). Species with the reductive pentose phosphate cycle or Calvin cycle incorporate atmospheric CO* directly into glycerate-3-P in the reaction catalyzed by ribulose-1,5-Pz carboxylase [1] These plants, including among others, spinach, soybeans, sunflower, and the algae, have been termed C-3 plants. Some species such as corn and sugarcane with the C-4 dicarboxylic acid pathway of photosynthesis (C-4 plants) synthesize malate and aspartate as their first stable products of CO2 assimilation with a subsequent release of COZ and reaction with ribulose-1,5-Pz carboxylase [2]. There are claims that the mesophyll cells contain both pathways of COs fixation and that sucrose, rather than malate or aspartate, is the compound linking the cell types [14] In this sequence, the chloroplast of the bundle sheath cells would serve primarily as a starch-storing organelle. We undertook to do preillumination experiments with corn and spinach leaves and to measure the levels of P-enolpyruvate, ribulose-1 , 5-P2, and glycerate-3-P during these experiments
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