Acclimation of photosynthesis to long-term elevated growth [CO 2 ], which includes shifts to a de-
crease in the carboxylation capacity; a decline in Rubisco activity, content, and transcript level; an accu-
mulation of nonstructural carbohydrates, especially starch; and a decrease of the N content in the plant, is
usually more marked when the supply of N to plants during growth is limited [51,55,60,61,154,155,
179,185,191,192]. With respect to plant growth, whereas elevated [CO 2 ] typically leads to a marked in-
crease in biomass in well-fertilized plants [46], this response changes with inadequate N fertilization
[154]. However, species-specific differences will be encountered under N-limiting growth conditions. El-
evated [CO 2 ] does not significantly enhance biomass of tobacco [155], rice [185], soybean [193], and sev-
eral woody species [194–196] when the N supply is limited. In other N-limited grown species, elevated
[CO 2 ] still increases plant biomass to some extent, but the stimulation is much less than in well-fertilized
plants [51,55,180,188,192,197–199]. Occasionally, large stimulation of growth by elevated [CO 2 ] under
N-limited conditions has also been observed [117,200].
IV. RISING CO 2 AND PLANT/LEAF DEVELOPMENTAL STAGE
The effect of elevated [CO 2 ] on plant growth also depends on plant age [154]. Most studies of the accli-
mation response under a CO 2 enrichment growth regime have focused on mature, fully expanded leaves.
However, there is strong evidence from the literature that there may be interactions between leaf ontogeny
and the degree of the acclimation response to elevated CO 2 exposure [50,65,93,154,201–205]. Leaves of
dicots, during their ontogeny, undergo two distinct photosynthetic phases: a phase of increasing assimi-
lation rates, which is correlated with import of nutrients and leaf expansion, and a prolonged senescence
phase of declining assimilation rates, with a transient peak of maximal assimilation rates between the two
phases [206]. In tobacco, both ambient (at 350 ppm) and high (at 950 ppm) CO 2 –grown plants exhibit this
photosynthetic pattern during leaf ontogeny; however, high CO 2 –grown plants have a temporal shift to
an earlier transition from the first phase of increasing photosynthesis to the senescence phase of declin-
ing photosynthesis [204]. These changes in photosynthetic rates are controlled largely by Rubisco activ-
ity, and the high CO 2 –grown leaves also enter the stage of photosynthetic decline several days before their
ambient CO 2 –grown counterparts [204]. Studies of the effects of elevated CO 2 on photosynthesis and Ru-
bisco in tomato during leaf ontogeny also reveal similar observations [50]. In addition, studies of other
C 3 annual species also show that long-term exposure to elevated [CO 2 ] leads to an enhancement of the
growth rate in young plants but not in older plants [207–209]. Similarly, for trees, increases in biomass
are mostly due to increased growth rates during the first year of elevated CO 2 exposure, and growth is en-
hanced less or not at all in the subsequent years [196,210,211]. Therefore, any consideration of elevated
[CO 2 ] effect on plant growth and physiology must also address time-dependent changes in the growth rate
of plants [154].
The expression of C 4 photosynthetic characteristics is controlled by factors such as leaf age and leaf
position. In some C 4 species, the first leaves show the normal C 3 type of photosynthesis, and this may
cause such species to be responsive to high CO 2 , at least in the short term [212]. In Portulaca oleracea,
an NADP-ME C 4 dicot, there is a shift in the route of CO 2 assimilation toward a limited, direct entry of
CO 2 into the PCR cycle in senescent leaves [213]. In Flaveria trinervia, also a C 4 dicot of the NADP-ME
subgroup, an estimated 10 to 12% of the CO 2 entered the PCR pathway directly in young expanding
leaves. However, CO 2 is apparently fixed entirely through the C 4 pathway in mature expanded leaves, and
this partitioning pattern is attributed to the bundle sheath compartment in young leaves, which have a rel-
atively high conductance to CO 2 [214].
In maize, an NADP-ME type monocot, pulse-chase experiments with mature and senescent leaf tis-
sues show that the predominant C 4 acids malate and aspartate differ between the two leaf ages [215]. Af-
ter a 10-sec chase, aspartate is the predominant C 4 acid in the mature leaves and malate is the major C 4 acid
in the senescent leaves. In addition, the activity of Rubisco during leaf ontogeny in maize parallels the de-
velopment in activity of this enzyme in C 3 plants [215]. Furthermore, a high CO 2 compensation point
(22–24 ppm) is found in senescent leaves of maize, in contrast to values of 0 to 10 ppm for most C 4 plants
[216]. Also in maize, the^14 C-labeling patterns of photosynthetic products in different sections of a devel-
oping leaf suggest that there may be some direct entry of CO 2 into the PCR pathway in the young tissues
of the basal section, whereas the C 4 pathway functions in the more differentiated tissues of the center and
top sections [217]. In addition, the activities of Rubisco and PEPC in maize leaves are found to vary ac-
RESPONSES TO RISING CO 2 AND CLIMATE CHANGE 45