CO2 Response in C4 Plants

Understanding the CO2-concentrating mechanism of C4 plants is becoming increasingly important as scientists strive to meet global needs for fuel and food. As in C3 photosynthesis, C4 plants use ribulose 1,5-bisphophate carboxylase/oxygenase (Rubisco) as the carboxylation enzyme for the formation of metabolic carbon. However, the initial reaction in C4 plants occurs in the mesophyll cells, where CO2 is fixed to pyruvate by phosphoenolpyruvate carboxylase (PEPc). The phosphoenolpyruvate formed in this initial reaction is converted to malate which is transferred from the mesophyll cells to the bundle sheath cells (Kanai and Edwards, 1999). This carboxylation reaction functions to increase Rubisco's efficiency by increasing the CO2 concentration ([CO2]) around Rubisco in the bundle sheath (Kanai and Edwards, 1999). The increased [CO2] inhibits the competitive photorespiratory reaction, thereby increasing the net carbon assimilation (von Caemmerer, 2000). C4 plants decrease the rate of photorespiration from approximately 50% in C3 plants (Björkman, 1966; Sharkey, 1988) to about 2% in C4 plants (Dai et al., 1993) at current atmospheric concentrations of CO2 and O2. In measured assimilation rates at different intercellular CO2 concentrations (Ci), the greater efficiency of the C4 is evident in the Ci that corresponds to a net positive rate as carbon assimilation or CO2-compensation point (Γ). Typically, C4 plants' Γ occurs at <10 µmolCO2 molair-1 whereas C3 plants' Γ often ranges from 40 - 50 µmolCO2 molair-1 (Lambers et al., 2006)

Biochemical limitations of different enzymes control the assimilation rates at different Ci. At low Ci, the photosynthetic rate is limited by the initial carboxylation of PEPc and can be given by:

A = C m V pmax C m + K p R m + g s C m

where Cm is the [CO2] in the mesophyll cells, Vpmax is the maximum rate of carboxylation of PEPc, Kp is the Michaelis-Menton constant of PEPc for CO2, Rm is the mesophyll dark respiration rate, and gsCm is the inward CO2 diffusion to the bundle sheath (von Caemmerer, 2000). At saturating light, the assimilation rate is linearly related to the maximum PEPc activity (von Caemmerer and Furbank, 1999).

Empirical gas-exchange measurements of the assimilation rate at very low Ci can be accomplished either by decreasing the [CO2] outside the leaf (Ca) to low levels or by driving stomatal conductance down, thereby decreasing the CO2 diffusion into the leaf. The first method is preferable since it will drive the Ci down, while other conditions within the leaf chamber can be optimized for measured gas exchange including high stomatal conductance. Errors in leaf stomatal conductance to water (gs) measurements are carried through when calculating Ci (see LI-COR, 2008). As the gs approaches 0, the uncertainty in the calculated Ci increases significantly. Conversely, greater gs decreases the uncertainty in the calculated Ci. Additionally, the greater gs will allow more CO2 to diffuse into the leaf for utilization in assimilation decreasing stomatal limitation (l).

Björkman, O. (1966) The effect of oxygen concentration on photosynthesis in higher plants. Physiologia Plantarum, 19, 618-633.

Dai, Z., Ku, M. & Edwards, G. E. (1993) C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration). Plant Physiology, 103, 83-90.

Kanai, R. & Edwards, G. E. (1999) The biochemistry of C4 photosynthesis. C4 Plant Biology (eds R. F. Sage & R. K. Monson). Academic Press, San Diego.

Lambers, H., Chapin III, F. S. & Pons, T. L. (2006) Plant Physiological Ecology. Springer Science+Business Media, LLC, New York.

LI-COR, Inc. (2008) Using the LI-6400/LI-6400XT portable photosynthesis system. LI-COR, Inc., Lincoln, NE, USA.

Sharkey, T. D. (1988) Estimating the rate of photorespiration in leaves. Physiologia Plantarum, 73, 147-152.

von Caemmerer, S. (2000) Biochemical Models of Leaf Photosynthesis. Techniques in Plant Science. CSIRO Publishing, Collingwood, VIC, Australia.

von Caemmerer, S. & Furbank, R. T. (1999) Modeling C4 photosynthesis. C4 Plant Biology (eds R. F. Sage & R. K. Monson). Academic Press, San Diego.

LI-COR Instruments for Measuring C4 Photosynthesis:

The LI-6400XT and LI-6800 Portable Photosynthesis Systems are compact, rugged, field portable instruments able to provide researchers with detailed information on plant responses, including the ability to measure fluorescence and gas exchange simultaneously. The LI-6800 is our newest system, featuring a touch-screen interface, improved fluorometer, better gas analyzer precision, better control over chamber conditions, and more.

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