REGULATION OF PHOTOSYNTHETIC CARBON ASSIMILATION IN LEAVES OF C3-C4 INTERMEDIATE SPECIES OF MORICANDIA AND FLAVERIA

The relationship between the gas-exchange characteristics, the content s 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 comp arison, in F. pringlei Gandoger (a C3 plant) and in F. bidentis (a C4 plant). Both C3-C4 intermediate species had pools of phosphoenolpyruva te, pyruvate, alanine and aspartate intermediate to those of the C3 an d C4 Species examined. Moricandia arvensis had large pools of glycine at low p(i), consistent with the operation of a glycine shuttle from m esophyll to bundle-sheath cells. It also had a high pool of triose-pho sphate at ambient partial pressures of CO2, indicating that a glycerat e-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-b isphosphate and triose-phosphate pool in M. arvensis, and a rise in th e pools of glycerate-3-phosphate and pyruvate in F. floridana, at low pi, show different patterns of metabolic regulation in M. arvensis and F. floridana at low p(i) in comparison to C3 and C4 plants.