mospheric [CO 2 ] [14], understanding the mechanisms of photosynthesis acclimation to rising [CO 2 ] and
other environmental stresses could potentially be translated into a basic framework for improving the ef-
ficiency of crop production in a future climate-changed world.
II. THE MAJOR PATHWAYS OF PHOTOSYNTHESIS
Present understanding of photosynthetic carbon metabolism classifies terrestrial plants into three major
photosynthetic categories: C 3 , C 4 , and Crassulacean acid metabolism (CAM). Each category possesses a
unique set of anatomical, physiological, and biochemical features that allows them to adapt to a specific
ecological niche [15]. It is estimated that approximately 95% of terrestrial plant species fix atmospheric
CO 2 by the C 3 (i.e., photosynthetic carbon reduction, or PCR) pathway, while 1% fix CO 2 by the C 4 path-
way and 4% by CAM [14].
A. The C 3 (Calvin) Cycle
In mesophyll cells of C 3 plants, CO 2 binding to its primary acceptor, ribulose-1,5-bisphosphate (RuBP),
is catalyzed by RuBP carboxylase/oxygenase (Rubisco), and the product of this carboxylation process, 3-
phosphoglycerate (PGA), is converted to other carbohydrates. In addition to the usual carboxylation re-
action, Rubisco catalyzes an oxygenase reaction in which O 2 reacts with RuBP to give PGA and phos-
phoglycolate, a process known as photorespiration [16]. The oxygenase reaction and associated
metabolism have an adverse effect on the efficiency of photosynthesis in C 3 plants, which results in a loss
of CO 2 , energy, and reducing potential [17]. The balance between carboxylation and oxygenation of
RuBP depends on the relative concentrations of CO 2 and O 2 at the site of Rubisco in the mesophyll
chloroplasts. A higher atmospheric [CO 2 ] will reduce photorespiration and therefore increase the leaf CO 2
exchange rate (CER) of C 3 plants (Figure 1).
B. The C 4 Pathway of CO 2 Fixation
C 4 plants have developed a biochemical mechanism to overcome the limitations of low atmospheric
[CO 2 ] and photorespiration [15,18–20]. In C 4 plants, atmospheric CO 2 is first hydrated to bicarbonate by
carbonic anhydrase in the cytosol of mesophyll cells; subsequently, it reacts with the three-carbon phos-
phoenolpyruvate (PEP) to give the C 4 acid oxaloacetate (OAA) in a reaction catalyzed by PEP carboxy-
lase (PEPC). OAA is rapidly converted to malate in the mesophyll chloroplasts by NADP–malate dehy-
drogenase (NADP-MDH) or transaminated to aspartate in the mesophyll cytosol by aspartate
36 VU ET AL.Figure 1 Photosynthesis of typical C 3 and C 4 plants versus ambient CO 2 concentration. Relative to C 3 plants,
C 4 plants have a low CO 2 compensation point (the intercept on the abscissa), a high carboxylation efficiency
(the initial slope of CO 2 -response curve), and a near-saturation photosynthetic rate at current atmospheric
[CO 2 ]. (Adapted from Refs. 15 and 31.)