Relationship Between Steady-State Gas Exchange, in vivo Ribulose Bisphosphate Carboxylase Activity and Some Carbon Reduction Cycle Intermediates in Raphanus sativus

Abstract

The relationships between CO2 assimilation rate, RuP2 carboxylase activity and sizes of the pools of ribulose 1,5-bisphosphate (RuP2) and 3-phosphoglyceric acid (PGA) were examined using a freeze clamp device to rapidly freeze sections of attached leaves of R. sativus which previously had gas-exchange measurements made on them. At high irradiance and ambient partial pressures of CO2 and O2, RuP2 carboxylase was fully active in vivo. Activity was less at very low CO2 pressures and at high CO2 pressures, particularly when combined with low O2 pressures. In vivo RuP2 carboxylase activity and both RuP2 and PGA pool sizes increased with increasing irradiance. RuP2 pool sizes were high at low CO2 pressures and decreased at high CO2 pressures. PGA pool sizes, on the other hand, were low at low CO2 and high at high CO2 pressures. A model of RuP2 carboxylase-oxygenase (Rubisco) kinetics is used to examine the quantitative relationship between in vivo RuP2 carboxylase activity, CO2 assimilation rate and RuP2 and PGA pools. The model predictions fit in vivo data, except at high CO2 pressures, if it is assumed that RuP2 does not bind tightly to the inactive enzyme form in vivo. It is shown that a large fraction of the RuP2 and PGA pools may be chelated by magnesium in the stroma and that the high RuP2 pools (e.g. at low irradiance) may represent an optimal concentration rather than be truly saturating. We conclude that RuP2 pool sizes above Rubisco site concentration do not necessarily indicate a Rubisco limitation of photosynthetic rate.

Light Adaptation/Acclimation of Photosynthesis and the Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Sun and Shade Plants

“…4). There was a significant linear relationship between these two parameters for each species, as has been demonstrated for a number of other species (3,8). However, the slope of the relationship for G. max (1.12) was approximately threefold higher than that for A. macrorrhiza (0.35).…”

“…These mechanisms, carbamylation-decarbamylation, Rubisco activase, and CA 1P metabolism, affect the efficiency of Rubisco use. At low PFDs, where the capacity for RuBP regeneration typically limits photosynthesis, the efficiency of Rubisco use is potentially low, as evidenced by the fact that the activity of the enzyme is generally reduced by these regulatory mechanisms to match the reduced capacity for RuBP regeneration (3,8). Plants …”

Modulation of Rubisco Activity during the Diurnal Phases of the Crassulacean Acid Metabolism Plant Kalanchoëdaigremontiana

“…We are uncertain whether the rise in intracellular pCO 2 per se or a factor associated with electron transport represents the dominant regulatory factor. In C 3 plants, carbamylation generally decreases in response to elevated CO 2 (Perchorowicz and Jensen, 1983;von Caemmerer and Edmondson, 1986), which results in decreased activation levels as a result of RuBP (von Caemmerer and Edmondson, 1986) and/or Pi limitation (Sharkey, 1985). This is a common process of photosynthetic control that balances carboxylation with substrate availability (von Caemmerer and Farquhar, 1981;Sharkey, Figure 5.…”

“…Carbamylation is light dependent (von Caemmerer and Edmondson, 1986;Hammond et al, 1998) and generally decreases in the presence of increased partial pressures of CO 2 (Perchorowicz and Jensen, 1983;von Caemmerer and Edmondson, 1986;Mate et al, 1993). The presence of tightbinding sugar phosphate inhibitors, including CA1P, may also be significant in the regulation of Rubisco activity (Keys et al, 1995;Parry et al, 1997).…”

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

“…Based on these results, we conclude that stimulation of light activation of rubisco by rubisco activase requires electron transport through PSI but not PSII, and that this light requirement is not to supply the ATP needed by the rubisco activase reaction. Furthermore, a pH gradient across the thylakoid membrane appears necessary for maximum light activation of rubisco even when ATP is provided exogenously.Light activation ofrubisco' has been demonstrated in leaves (15,18,25,29) and intact chloroplasts (2, 10). The light responses of the activation of rubisco, the enzyme which catalyzes photosynthetic CO2 assimilation (16), and the rate of CO2 assimilation have been shown to proceed in tandem (18,25,29).…”