Photosynthetic path of carbon dioxide in spinach and corn leaves.

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) COZ 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* in the dark or in the light subsequent to illumination in the absence of CO, in order to accumulate the primary compound for carboxylation. Prior illumination of corn and spinach leaves in an atmosphere of Nz containing 1.5 % 02, enhanced dark 14C02 fixation 15to 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 14C02 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 Nz and 1.5 % 02, after pulses of CO* 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 COZ. Glycerate 3-phosphate was little affected during preillumination but increased rapidly upon addition of COz. In both species the level of phosphoenolpyruvate was essentially unchanged during preillumination but increased sharply in corn leaves with introduction of CO 2. CO* 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 COZ in detached leaves of C-4 (corn) and C-3 plants (spinach).