Carbon Metabolism and Gas Exchange in Leaves of Zea Mays L. : Changes in CO2 Fixation, Chlorophyll a Fluorescence and Metabolite Levels During Photosynthetic Induction

Changes in the rate of CO2 uptake, chlorophyll a fluorescence and contents of metabolites were measured during illumination and darkening of maize leaves. Upon illumination, the contents of aspartate and alanine declined rapidly and there were rapid increases in the contents of 3-phosphoglycerate and triose phosphates. The amounts of pyruvate and phosphoenolpyruvate increased much more slowly. Upon darkening, the levels of 3-phosphoglycerate, phosphoenolpyruvate and triose phosphates fell sharply, while the amount of pyruvate increased. It is proposed that metabolite gradients in C4 photosynthesis are built-up during induction through interchange of carbon between amino acids, metabolites of the C4 pathway and 3-phosphoglycerate and triose phosphates, since CO2 fixation during the first 5 min of photosynthesis was insufficient to account for the observed build-up of intermediates. Changes in the rates of CO2 uptake and chlorophyll a fluorescence quenching are discussed in the light of the changes in metabolites.