Highly characteristic responses of thylakoid membranes were observed in function and composition when fully developed plants of Amaranthus hypochondriacus L. grown under light sufficient (2000 μmol m-2 s-1) conditions were transferred to light limited conditions (650 μmol m-2 s-1 and 200 μmol m-2 s-1). The whole-chain, photosystem I and photosystem II electron transport rates were depressed in both bundle sheath and mesophyll thylakoids with remarkable differences between them in variation of rates under limiting light. The reduction in PSI electron transport in the mesophyll could be attributed to reduced PSI centres, while in the bundle sheath, a modulation of cytochrome b6/f complex regulated the rates of PSI electron transport. The requirement for an unaltered number of PSI centres under limiting light in the bundle sheath is ascribed to operation of an energy-consuming C4 pump.
Illumination Increases the Affinity of Phosphoenolpyruvate Carboxylase to Bicarbonate in Leaves of a C4 Plant, Amaranthus hypochondriacus