aminotransferase, depending on the C 4 acid-decarboxylating mechanism of the C 4 plant [21]. These C 4
compounds are then transported to the bundle sheath cells, where they are decarboxylated to release CO 2
by one of the three C 4 acid-decarboxylating enzymes: NADP-malic enzyme (NADP-ME), NAD–malic
enzyme (NAD-ME), or PEP carboxykinase [18,19,22].
In the NADP-ME species, which contain crops of global importance including sugarcane, maize, and
sorghum, OAA is reduced in the mesophyll chloroplasts via NADP-MDH to malate, which is then trans-
ferred to the adjacent bundle sheath cells. In the bundle sheath chloroplasts, malate undergoes decar-
boxylation catalyzed via NADP-ME to produce CO 2 which is reassimilated by Rubisco in the conven-
tional Calvin C 3 (PCR) cycle. In C 4 species in which NAD-ME is the major C 4 acid-decarboxylating
enzyme (e.g., Atriplex spongiosa, Portulaca oleracea, Amaranthus edulis), aspartate from the mesophyll
cells enters the bundle sheath mitochondria, where it is converted to OAA. OAA is then reduced to
malate, which, in turn, is decarboxylated via NAD-ME, generating CO 2 to be assimilated by the PCR cy-
cle. In species in which PEP carboxykinase is the primary decarboxylating enzyme (e.g., Panicum maxi-
mum, Chloris gayana, Sporobolus fimbriatus), aspartate from the mesophyll cells is converted to OAA in
the bundle sheath cytosol, and OAA is subsequently decarboxylated producing CO 2 , which is then as-
similated by the PCR cycle.
Thus, the reactions that are unique to C 4 photosynthesis can be considered as an additional step to
the conventional C 3 pathway. They operate to transfer CO 2 from mesophyll to bundle sheath cells through
the intermediary of a dicarboxylic acid and consequently increase levels of CO 2 in bundle sheath cells
specifically for refixation via Rubisco in the C 3 cycle [19]. Through this additional metabolic pathway,
C 4 plants are able to concentrate CO 2 in the Rubisco-containing bundle sheath cells to levels up to 3 to 20
times higher than atmospheric [CO 2 ] [19,23–25]. Photosynthesis by C 4 plants is therefore near saturation
at current atmospheric [CO 2 ], and a rise in atmospheric [CO 2 ] presumably may have little or no effect on
C 4 photosynthesis (Figure 1).
C. The CAM Pathway
CAM (Crassulacean acid metabolism) is a photosynthetic process, named after the family Crassulaceae,
in which the accumulation of malic acid in the dark, a distinctive property of the process, was first ob-
served [26]. CAM plants are widely distributed in arid and semiarid regions, where their contribution to
community biomass production is significant [26–28]. Although many plants that exhibit CAM belong to
the dicotyledonous Crassulaceae family (Kalanchoespp.,Sedumspp.), this photosynthetic process is also
widespread in plants of other dicotyledonous families (Aizoaceae, Asclepiadaceae, Bataceae, Cactaceae,
Caryophyllaceae, Chenopodiaceae, Compositae, Convolvulaceae, Euphorbiaceae, Plantaginaceae, Portu-
lacaceae, Vitaceae) as well as the monocotyledonous families (Agavaceae, Bromeliaceae, Liliaceae, Or-
chidaceae) and even the Pteridophyte family (Polypodiaceae) [18].
CAM plants normally close their stomata during the day to prevent water loss. At night, the stomata
are open, and atmospheric CO 2 enters the cytoplasm of chloroplast-containing cells of photosynthetic leaf
or stem tissues and combines with PEP, a product of glucan metabolism, via PEPC to form OAA [18,29].
OAA is subsequently reduced by NAD–malate dehydrogenase to malate, which then accumulates in large
vacuoles that are characteristic of the cells of CAM plants. During the daylight hours, stomata become
closed, and malate is transported back into the cytoplasm, where it is decarboxylated by an NADP–malic
enzyme. The CO 2 just released enters the chloroplasts, where it is fixed by Rubisco of the conventional
C 3 cycle. Although CAM plants and C 4 plants share the two major carboxylating enzymes PEPC and Ru-
bisco, the carbon reduction catalyzed by these enzymes differs temporally and spatially, respectively, for
these two photosynthetic categories [26,28]. Furthermore, the Km(PEP) value of PEPC from CAM plants
is less than one third that of C 4 plants [26]. Thus, the effects of elevated atmospheric [CO 2 ] on the uptake
of CO 2 by CAM plants can be different than for C 4 plants [30].
III. RISING ATMOSPHERIC CO 2 AND ITS INTERACTIONS WITH
OTHER ENVIRONMENTAL VARIABLESA. Plant Responses to Rising CO 2
Research during the past 20 years on growth, as well as mechanisms and acclimation (down-regulation
or up-regulation) in photosynthetic processes, as a result of long-term exposure to elevated [CO 2 ], has fo-
RESPONSES TO RISING CO 2 AND CLIMATE CHANGE 37