D. Rising CO 2 and Light Intensity
Measurements of CO 2 enrichment effects on photosynthesis have usually been carried out with relatively
high irradiance. In nature, photosynthesis occurs in both high and low light environments, and light-lim-
ited photosynthesis can account for half of the total carbon gain [33]. Several studies show that C 3 pho-
tosynthesis is enhanced by elevated [CO 2 ] even under light-limited conditions [62,171–174], and the en-
hancement rises with temperature [33]. Photosynthesis versus solar irradiance response curves show that
soybean leaves grown and measured at 660 ppm CO 2 have lower light compensation points, steeper ini-
tial slopes, higher apparent quantum yields, and greater CER at light saturation than those adapted to and
measured at 330 ppm CO 2 [172]. Canopy photosynthesis of soybean increases linearly with increases in
growth [CO 2 ] from 160 up to 990 ppm as a result of improvements in leaf area index, leaf photosynthe-
sis, and quantum yield [173].
Most studies of leaf photosynthetic acclimation to elevated CO 2 have focused on the most recently
fully expanded, sunlit leaves. This may not reliably predict acclimation of the whole canopy at one spe-
cific crop developmental stage, as a difference in acclimation could occur between the uppermost, fully
developed sunlit leaves and the older, shaded leaves located deeper within the canopy [65,66,89]. Stud-
ies of photosynthetic acclimation in a vertical profile of leaves through canopies of wheat [65,89] and sun-
flower [66] show that even at stages of development at which elevated CO 2 did not affect the carboxyla-
tion capacity in the uppermost fully expanded leaves, there was a decrease in the lower shaded leaves.
In a crop canopy, photosynthesis is light limited for all of the day for the interior or lower canopy
leaves [33]. For a citrus canopy, although the absolute benefits of elevated CO 2 are greatest at high light
intensity, the relative benefits are more significant at low light levels [175]. The positive direct effect of
elevated growth CO 2 on citrus photosynthesis more than compensates for the negative self-shading effect
produced by the high CO 2 -induced proliferation of leaf area [175].
It is expected that the interaction between different growth CO 2 concentrations and light intensities
will be different for C 3 and C 4 plants, as the C 4 photosynthetic pathway requires more energy than the C 3
pathway [62]. This extra energy is associated with the regeneration of PEP by the C 4 pathway in the mes-
ophyll cells [176]. Studies with various species of C 4 weed grasses at elevated [CO 2 ] showed that growth
at low light did not increase the growth-enhancing effects of CO 2 enrichment, whereas elevated [CO 2 ] and
high growth irradiance significantly enhanced their net photosynthesis and early growth [177]. Assimila-
tion-irradiance relationships for plants grown at ambient and elevated [CO 2 ] indicate that CO 2 -enriched
plants had higher light saturation values and greater rates at high irradiance levels [177]. Studies of Pan-
icumspecies with different photosynthetic pathways showed that twice-ambient growth [CO 2 ] enhanced
biomass at both low and high irradiance regimes for the C 3 (P. laxum) grass but only at high light for the
C 4 (P. antidotale) species [62]. The elevated CO 2 –grown C 3 plants had more leaves, greater total leaf area,
longer main stems, more nodes, and more tillers than the ambient CO 2 –grown plants under both low and
high light treatments. These enhancements in biomass were not seen for the elevated CO 2 –grown C 4
species under low light regimes. Only under high light did elevated CO 2 enhance stem elongation and shoot
biomass accumulation in the C 4 plants [62]. In addition, there was no significant difference in leaf photo-
synthetic rates, measured at respective growth [CO 2 ], between the ambient and elevated CO 2 –grown plants
for both P. laxumandP. antidotale, although small but nonsignificant enhancements by elevated growth
CO 2 were observed for the low light–treated C 3 and the high light–treated C 4 plants [62].
E. Rising CO 2 and Nitrogen Nutrition
As CO 2 is just one of many inorganic substrates required by plants, long-term response of plant photo-
synthesis and growth to elevated [CO 2 ] also depends on the availability of mineral nutrients and the way
in which plants utilize them [154]. Most studies of elevated CO 2 and nutrient interactive effects have fo-
cused on nitrogen (N), because it is a common limitation in many natural and agroecosystems [178].
There have been many reports on the interaction between N nutrition and the response of photosynthesis,
metabolism, and growth to elevated CO 2 [154,155,179–187]. In plants grown under elevated [CO 2 ], the
overall N concentration usually decreases [55,60,188–190]. This overall N decrease under elevated
growth [CO 2 ] might reflect either a higher N use efficiency due to reallocation of proteins, an ontogenetic
drift leading to accelerated senescence as a result of faster growth, or inadequate N fertilization, uptake,
and/or assimilation [154].
44 VU ET AL.