Photosynthetic carbon metabolism in Panicum milioides, a C 3-C 4 intermediate species: Evidence for a limited C 4 dicarboxylic acid pathway of photosynthesis

Abstract

Panicum milioides, a naturally occurring species with C 4-like Kranz leaf anatomy, is intermediate between C 3 and C 4 plants with respect to photorespiration and the associated oxygen inhibition of photosynthesis. This paper presents direct evidence for a limited degree of C 4 photosynthesis in this C 3-C 4 intermediate species based on: 1. (a) the appearance of 24% of the total 14C fixed following 4 s photosynthesis in 14CO 2-air by excised leaves in malate and aspartate and the complete transfer of label from the C 4 acids to Calvin cycle intermediates within a 15 s chase in 12CO 2-air; 2. (b) pyruvate- or alanine-enhanced light-dependent CO 2 fixation and pyruvate stimulation of oxaloacetate- or 3-phosphoglycerate-dependent O 2 evolution by illuminated mesophyll protoplasts, but not bundle sheath strands; and 3. (c) NAD-malic enzyme-dependent decarboxylation of C 4 acids at the C-4 carboxyl position, C 4 acid-dependent O 2 evolution, and 14CO 2 donation from [4- 14C]C 4 acids to Calvin cycle intermediates during photosynthesis by bundle sheath strands, but not mesophyll protoplasts. However, P. milioides differs from C 4 plants in that the activity of the C 4 cycle enzymes is only 15 to 30% of a C 4 Panicum species and the Calvin cycle and phospho enolpyruvate carboxylase are present in both cell types. From these and related studies (Rathnam, C.K.M. and Chollet, R. (1979) Arch. Biochem. Biophys. 193, 346–354; (1978) Biochem. Biophys. Res. Commun. 85, 801–808) we conclude that reduced photorespiration in P. milioides is due to a limited degree of NAD-malic enzyme-type C 4 photosynthesis permitting an increase in pCO 2 at the site of bundle sheath, but not mesophyll, ribulosebisphosphate carboxylase-oxygenase.