The aim of this work was to investigate how light regulates the activity of phosphoenolpyruvate carboxylase in vivo in C4 plants. The properties of phosphoenolpyruvate carboxylase were investigated in extracts which were rapidly prepared (in less than 30 seconds) from darkened and illuminated leaves of Zea mays. Illumination resulted in a significant decrease in the So.5(phosphoenolpyruvate) but there was no change in V,,,. The form of the enzyme from illuminated leaves was less sensitive to malate inhibition than was the form from darkened leaves. At low concentrations of phosphoenolpyruvate, the activity of the enzyme was strongly stimulated by glucose-6-phosphate, fructose-6-phosphate, triose-phosphate, alanine, serine, and glycine and was inhibited by organic acids. The enzyme was assayed in mixtures of metabolites at concentrations believed to be present in the mesophyll cytosol in the light and in the dark. It displayed low activity in a simulated 'dark' cytosol and high activity in a simulated 'light' cytosol, but activities were different for the enzyme from darkened compared to illuminated leaves. PEP2 carboxylase occurs widely, perhaps universally, in cells of higher plants (11), but in plants with C4 and CAM modes of photosynthesis, specific isoenzymes are present in the cytosol of photosynthetic tissues at very much higher activities than in C3 photosynthetic or in nonphotosynthetic tissues (11,26). In C4 plants, PEP carboxylase is the initial step of the 'CO2 pump' which transfers CO2 from mesophyll cells and concentrates it in the bundle-sheath cells. Not only must the CO2 pump be coordinated with the rate of Calvin cycle turnover, but PEP consumption is also likely to be regulated so that photosynthesis can proceed efficiently at different irradiances, temperatures, and CO2 concentrations, and particularly to ensure that glycerate-3-P is not drained into the C4 cycle during photosynthesis (6). Since PEP also lies at an important branch-point in plant metabolism its utilization by PEP carboxylase must also be curtailed in darkness.There is good evidence that light modulation of PEP carboxylase occurs in vivo. For example, PEP has been observed to increase transiently upon illumination of maize leaves (6) (19,29,30).The evidence for regulation of the enzyme directly by light in C4 plants has, until recently, been weak. There have been reports of 2-fold changes in Vm> and a reduction in K,,,, but these changes vary between species (9, 10). These reports can be criticized on the grounds that the extraction procedure lasted 15 min and that any light-induced changes might by then have been reversed. However, Huber and Sugiyama (8) have recently reported that a change in the sensitivity of PEP carboxylase to effectors occurs during light-dark transitions. The evidence for metabolite modulation of PEP carboxylase is altogether stronger. Unlike PEP carboxylases from C3 and CAM plants, PEP carboxylase from C4 plants has a relatively low affinity for PEP (25,26), but in common with them, the enzyme f...

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Parallel measurements of contents of photosynthetic intermediates, activities of enzymes of photosynthetic carbon assimilation, gas-exchange rates and components of chlorophyll-fluorescence quenching in leaves of C 4 plants are considered in relation to changes in photon flux density (PFD) and CO 2 . The influence of varying light and CO 2 concentration upon changes in the amounts of phosphoenolpyruvate (PEP) in leaves of C 4 plants during steady-state photosynthesis are interpreted in terms of the regulatory properties of PEP carboxylase and in terms of feedback interactions between the Calvin cycle and the C 4 cycle. Relations between electron transport and carbon assimilation are discussed in terms of the regulation of the supply of ATP and NADPH and the demands of carbon assimilation. In low light these relations differ in C 3 and C 4 plants. The lag in photosynthetic carbon assimilation in maize that follows a decrease in PFD has been analysed. The changes that occur in enzyme activities, metabolites and components of chlorophyll-fluorescence quenching following the transition from high to low light indicate that diminished production of ATP and NADPH is responsible for the lag in photosynthetic carbon assimilation and may reflect a stimulation of cyclic electron flow to make up a deficit in ATP.

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