The photosynthetic carbon metabolism of Zea mays and Gomphrena globosa: the location of the CO2 fixation and the carboxyl transfer reactions

Zea mays and Gomphrena globosa form labeled aspartate and malate (C4-acids) via β-carboxylation of P-enolpyruvate during photosynthesis. Studies of the redistribution of 14C in pulse- and chase-type feedings of 14CO2 indicate that most labeled phosphorylated compounds are formed from the C4-acids. A mechanism involving CO2 as a transitory intermediate is advanced to explain the carboxyl transfer from the C4-acids to 3-phosphoglyceric acid (3-PGA). In this model, CO2 is generated through the oxidative decarboxylation of malic acid by "malic" enzyme, and is refixed by RuDP carboxylase to form 3-PGA. The pattern of labeling of photosynthetic products, the extractable enzyme activities, and the gas exchange properties of these plants appear to be consistent with this proposed sequence of reactions. The location of 14C-labeled compounds was determined by radioautography, and by nonaqueous density gradient separation. Differential grinding was used to study the location of some photosynthetic enzymes. These indicate that CO2 fixation by β-carboxylation occurs in the leaf mesophyll. The carboxyl transfer and the reactions leading to the photosynthesis of starch appear to be confined predominantly to the bundle sheath cells. Rapid transport of C4-acids from the site of CO2 fixation in the mesophyll to the bundle sheath may occur by plasmodesmata.