cording to leaf position, with activity of PEPC less than that of Rubisco in the lower leaves, whereas the
upper leaves exhibit high levels of PEPC [218]. Moreover, in maize, bundle sheath cell walls of young and
senescent leaves have a relatively high conductance, leading to a low capacity for CO 2 concentration in
these bundle sheath cells during photosynthesis [219]. In the uppermost fully expanded leaves of sugar-
cane, CER, stomatal conductance, and activities of both PEPC and Rubisco increase from the base to the
tip of the leaf [220]. Analyses of a range of leaf developmental stages in maize also indicate that when leaf
chlorophyll and Rubisco protein contents are below a critical level, i.e., 50% or less compared with those
found in mature leaves, the degree of photorespiration could approach that of C 3 plants [219].
In C 4 monocots and dicots, the vascular system features a radial pattern structure (Kranz type) around
which photosynthetic bundle sheath and mesophyll cells are arranged [23,221,222]. Such compartmen-
talization for metabolic cooperation between mesophyll and bundle sheath cells is essential for the C 4
pathway. In C 4 dicots, bundle sheath cells generally have centripetally arranged chloroplasts, whereas in
monocots the arrangement of the chloroplasts varies with the C 4 acid-decarboxylating enzyme subtype:
centrifugal for NADP-ME species, centripetal for NAD-ME species, and random for PEP carboxykinase
species [23,34]. In maize and sugarcane, chloroplasts of the bundle sheath and mesophyll cells are mor-
phologically similar early in development; i.e., both contain granal stacks [34,223,224]. However, subse-
quent dedifferentiation of bundle sheath cell chloroplasts results in the agranal bundle sheath chloroplasts
as seen in the mature leaves [223,224]. With respect to leaf ontogeny, leaf shape results from distinct pat-
terns of cell division and expansion in both shoot apical meristem and leaf primordium [34]. Leaves of
monocots are derived from the outer two layers of the shoot apical meristem, whereas those of dicots are
derived from the outer three layers of the shoot apical meristem. The shape of monocot leaves is gener-
ated through polarized patterns of cell division and expansion that maintain cells in files. In maize, cell
divisions occur throughout the leaf and become restricted to the leaf base only after initiation of the ligule
at the boundary of leaf blade and leaf sheath. Dicot leaves, which are generally less uniform in shape than
monocot leaves, are generated through less polarized divisions [34].
The expression of C 4 genes does not occur until Kranz anatomy has been established, and exclusive
use of the C 3 photosynthetic pathway may occur prior to the full differentiation of Kranz anatomy [34].
Therefore, one of the proposed explanations for the biomass enhancement observed in C 4 plants grown at
elevated CO 2 is that the “immature” C 4 pathway in young C 4 leaves has C 3 -like characteristics, and thus
photosynthesis of these young leaves is responsive to increasing CO 2 above current ambient levels
[32,62,177,212]. This hypothetical explanation, however, may be species specific, as one study argues
against this possibility by showing that the gas exchange parameters in young leaves of Panicum antido-
tale(C 4 , NADP-ME) and Panicum coloratum(C 4 , NAD-ME) do not have C 3 -like characteristics [225].
V. CONCLUSION
In the 21st century, world agriculture is confronted with unprecedented environmental challenges. Ero-
sion of the protective ozone layer, increased ultraviolet B (UV-B) irradiation, desertification, damage to
long-established ecologies, greenhouse effects of rising atmospheric [CO 2 ] and temperature, and shifts in
regional scale rainfall patterns [12,226] are all environmental concerns that will affect global agriculture
on a scale never before encountered. A change in global climate and a rapidly expanding world popula-
tion accelerate the demand for food, energy, and fresh water and threaten the ability of the world to feed
itself [13,227–229]. As a consequence, the need to enhance the production efficiency of agricultural crops
and their tolerance to warmer, more arid environmental conditions will escalate as competition for arable
land and fresh water increases. However, we do not know what the net consequences of plant responses
to these environmental changes will be, simply because we do not understand enough about how plants
grow and their interactions with the environment to predict the effects of such changes [12,69]. There-
fore, producing crops under climate change conditions will be a growing challenge in world agriculture.
It has been well recognized that increasing crop yields require an increase in photosynthesis, and ge-
netic manipulation of photosynthetic processes has been the primary focus for crop improvement
[13,229–233]. Thorough knowledge of crop growth and development and plant interactions with the en-
vironment, as well as new approaches and ambitious strategies, such as “reengineering” photosynthesis
[13,229,230,233,234], “remodeling” Rubisco for more effectiveness [231], or “supercharging” photo-
synthesis of C 3 crop plants with C 4 genes [235–237], may all be required to improve crop efficiency at
turning atmospheric carbon into food and maintaining world food supplies and nutrition.
46 VU ET AL.