The photosynthetic electron transport in thylakoids of the facultative CAM plant Mesembryanthemum crystallinum (L.)

摘要

Crassulacean acid metabolism (CAM) is characterized by a circadianally varying demand for ATP relative to NADPH. In addition, the absolute demand for electrons could fluctuate with the availability of CO_2. In CAM phases II and IV (nomenclature according to Osmond 1978) CO_2 is fixed in a C_3 like manner and the whole chain electron transport in thylakoids is considered to be sufficient to deliver ATP and NADPH in the required stochiometric ratio of 1.2 to 1.5. In the malate decarboxylation phase (CAM phase III) one pyruvate per reduced CO_2 is regenerated to hexoses by gluconeogenesis (Holtum and Osmond 1981, Winter and Smith 1996). In this cycle, the pyruvate-P-dikinase (PPDK) requires 2 extra ATP. As in M. crystallinum the PPDK is located in the chloroplasts (Kondo et al. 1998), the plastidic demand for ATP, relative to NADPH, transiently increases to a value of 1.6 to 2.0. This extra demand for ATP cannot be covered by linear electron flow. In this phase, chloroplasts need an additional source of ATP, otherwise, the photosynthetic carbon cycle and related reactions would become limited by ATP availability. This would diminish the efficiency of hotosynthetic metabolism and provide the danger of photodynamic damage in high light.