Light-Enhanced Dark ¹⁴CO₂ Fixation by Leaves in Relation to the C₄ Dicarboxylic Acid Pathway of Photosynthesis

Abstract

The technique of light-enhanced dark carbon dioxide fixation has been employed to critically examine the primary route of CO₂ incorporation in leaves of two C₄ plants (Setaria italica and Amaranthus paniculatus) in comparison with that of a C₃ plant (Rumex vesicarius). Prior illumination of detached leaves enhanced their dark ¹⁴CO₂ fixation five- to tenfold over that of unilluminated leaves. No variation was observed among C₃ and C₄ plants, in the conditions required for their light-enhanced dark fixation. Fifty to sixty percent of initially incorporated radioactivity by the leaves of R. vesicarius was in 3-phosphoglycerate. The leaves of S. italica and A. paniculatus fixed most of the radioactivity into the C₄ acids malate and aspartate. Sugar phosphates, including 3-phosphoglycerate, were not labelled at all. Our data confirm that, in C₄ plants, CO₂ is initially fixed through phosphoenolpyruvate carboxylation and thus disprove earlier claims that ribulose bisphosphate can be the primary acceptor of CO₂ in a C₄ plant. The observations further suggested that aspartate formation during carbon fixation might be favoured by illumination.