Regulation of photosynthetic carbon assimilation in leaves of C₃-C₄ intermediate species of Moricandia and Flaveria

The relationship between the gas-exchange characteristics, the contents of photosynthetic intermediates and the quantum yield of photosystem II was examined at different intercellular partial pressures of CO₂ (p_i) in attached leaves of Moricandia arvensis L. (D.C.) and Flaveria floridana J.R. Johnson (both C₃-C₄ intermediate plants) and, for comparison, in F. pringlei Gandoger (a C₃ plant) and in F. bidentis (a C₄ plant). Both C₃-C₄ intermediate species had pools of phosphoenolpyruvate, pyruvate, alanine and aspartate intermediate to those of the C₃ and C₄ species examined. Moricandia arvensis had large pools of glycine at low p_i, consistent with the operation of a glycine shuttle from mesophyll to bundle-sheath cells. It also had a high pool of triose-phosphate at ambient partial pressures of CO₂, indicating that a glycerate-3-phosphate/triose-phosphate shuttle could operate in this species. This was not the case in F. floridana. A decline in the ribulose-1,5-bisphosphate and triose-phosphate pool in M. arvensis, and a rise in the pools of glycerate-3-phosphate and pyruvate in F. floridana, at low p_i, show different patterns of metabolic regulation in M. arvensis and F. floridana at low p_i in comparison to C₃ and C₄ plants.