Analysis of the control of photosynthesis in C ₄ plants by changes in light and carbon dioxide

Abstract

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.

Changes in Activities of Enzymes of Carbon Metabolism in Leaves during Exposure of Plants to Low Temperature

“…Because the activation of Rubisco, stromal Fru 1,6-P2ase, and NADP-MDH is dependent on reducing power from electron transport, it is important to note that the almost complete activation of these enzymes in spinach occurs under conditions that reduce the efficiency of linear electron transport. A restriction in carbon assimilation, as would occur at low temperature, usually leads, through photosynthetic control, to an inactivation of redox-regulated enzymes (18). However, the decrease in the ratio triose-P/ glycerate-3-P suggests that carbon assimilation is restricted relatively less than electron transport after transfer to low temperature.…”

“…There was an increase in glycerate-3-P, but the amount of triose-P remained essentially constant. Therefore, the ratio of triose-P/glycerate-3-P decreased, suggesting a restriction in the ability of the leaf to generate the ATP and/or NADPH needed to reduce glycerate-3-P to triose-P (18). The amounts of Fru 1,6-P2 and Sed 1,7-P2 increased (the latter by nearly eightfold).…”

Can the progressive increase of C4 bundle sheath leakiness at low PFD be explained by incomplete suppression of photorespiration?

“…For example, interconversion of PGA and PEP was shown to occur in maize (Furbank & Leegood 1984) and presumably functions to balance the amount of 3‐carbon compounds in the C 4 cycle with the photosynthetic flux through the complete pathway. Although there are certainly plenty of examples of light regulation of key enzymes in the C 4 pathway, the magnitude of the photosynthetic flux under low PFD is supposed to be controlled by ATP supply (Leegood et al . 1989).…”

“…(1992) hypothesized incomplete suppression of photorespiration at low PFD to cause φ to increase. At low PFD, ATP can be considered limiting (Leegood et al . 1989) and fluxes through C 4 and Calvin‐Benson‐Bassham (CBB) cycles decrease.…”

Bundle Sheath Leakiness and Light Limitation during C4 Leaf and Canopy CO2 Uptake

“…The underlying mechanisms have been analyzed extensively. Based on studies of photosynthetic behavior in C 4 plants, Leegood et al (1989) argued that the energy supply for the assimilatory pathway (production of ATP and NADPH) declines drastically following a transition from high to low PFD. Stimulation of cyclic electron flow and Q-cycle activity (Furbank et al, 1990) might make up for part of the ATP shortage (by cyclic par- Figure 5C, using Eq.…”