tropical grasslands, many temperate grasslands, and most disturbed landscapes in warm regions, and 8 of
the world’s 10 most invasive weeds are C 4 species [95,99,100].
In C 4 species, the presence of a CO 2 -concentrating mechanism has led to a general assumption that
there would be little or no increase in photosynthesis and growth with rising atmospheric [CO 2 ]. How-
ever, examination of the literature reveals a positive growth response to a doubling of [CO 2 ] for a num-
ber of C 4 plants, although to a smaller extent (~14%) than for C 3 plants (40–58%)
[31,32,36,37,101–105]. Therefore, it cannot be assumed that C 4 species will not respond to rising at-
mospheric CO 2. In spite of the growth stimulation, these C 4 plants often show little or no enhancement
in leaf CER at elevated [CO 2 ], which is in contrast to C 3 species [31,37,62,105–107]. Chamber-grown
C 4 maize and sugarcane showed no differences in leaf photosynthetic rates between the ambient CO 2 –
and double-ambient CO 2 –grown plants, although leaf area and total plant biomass of the CO 2 -enriched
plants increased 14% [104]. Also, maize plants grown in controlled chambers under a triple-ambient
CO 2 atmosphere (1100 ppm) were only 10% higher in light-saturated rates of photosynthesis for ma-
ture leaves but 20 to 23% higher in total biomass and leaf area [108]. In a study conducted in naturally
sunlit temperature-gradient greenhouses to investigate the effects of elevated CO 2 and high tempera-
tures on growth and photosynthesis of sugarcane (cv. CP 73-1547), [CO 2 ] at 700 ppm increased leaf
area by 31% (Figure 2A), total aboveground dry weight by 21% (Figure 2B), and main stem juice vol-
ume by 83% (Figure 2C) when compared with plants grown at 360 ppm [CO 2 ] [109,110]. Furthermore,
growth of sugarcane plants under both elevated CO 2 (700 ppm) and temperature (4.5°C above baseline
temperature control, which was 2°C above outside ambient) increased leaf area by 56%, total above-
ground dry weight by 74%, and juice volume by 164% (Figure 2A, B, C). These increases occurred
without an enhancement of leaf CER, measured at the growth [CO 2 ] for the most expanded sections of
the uppermost, fully expanded leaves (Figure 2D).
Causes of the observed growth stimulation by elevated CO 2 on C 4 plants remain uncertain, but fac-
tors that indirectly impinge on Rubisco may be involved in this enhanced growth [32,46]. First, a reduc-
tion in stomatal aperture and conductance is a common response to a doubling of atmospheric growth
[CO 2 ]. This decrease occurs across a variety of C 3 and C 4 species, although there are cases of insensitive
stomatal responses [14,33,46]. The reduction in stomatal aperture and conductance explains the reduction
in transpiration observed in plants grown under elevated [CO 2 ]. This results in an improved water use ef-
ficiency (WUE) and tissue water status and a potentially increased growth and/or yield with no additional
penalty in water consumption [33,111]. Under water-shortage conditions, an improvement in WUE in-
duced by elevated CO 2 could delay soil drying and reduce drought inhibition of C 4 vegetation and thus
enhance growth, and this has also been suggested as a factor in the improved photosynthesis and increased
biomass of some C 4 species [112–115]. Second, adverse growth conditions such as low nitrogen, high
salinity, or limited soil water availability may undermine the effectiveness of the CO 2 -concentrating
mechanism by increasing CO 2 leakage from the bundle sheath, thus making C 4 species more responsive
to elevated atmospheric CO 2 [116,117]. Even a small, but consistent, percent stimulation in the CO 2 as-
similation rate throughout the growth season could account for the growth enhancement seen in the C 4
species [37,62]. Third, elevated growth [CO 2 ] can enhance tillering and leaf area, so that total plant pho-
tosynthesis is greater, even without an increase in CO 2 uptake rate per unit leaf area [14,39,46,118]. In
addition, changes in dark respiration and photosynthate partitioning, which are still poorly understood for
CO 2 -enriched C 4 species, may explain part of the enhanced growth [37]. In maize, the increased capacity
to synthesize and utilize sucrose and starch to produce extra energy by respiration could contribute to
plant biomass enhancement under elevated growth [CO 2 ] [108].
- CAM Species
The response of CAM plants to elevated atmospheric [CO 2 ] is less clear because studies examining the
CO 2 -enrichment responses of CAM plants are limited, with varying results being reported. Presumably,
minimal response may be expected for plants that are capable of raising their daytime internal CO 2 lev-
els as high as 10,000 ppm through decarboxylation of the C 4 malic acid accumulated during the previ-
ous evening period; however such a presumption is only partially corroborated [14,26]. Under a dou-
bling of atmospheric [CO 2 ], there was no enhancement in leaf CER and leaf area or total plant biomass
for pineapple, an economically important CAM species, but these parameters were 20 to 44% higher
forAechmea magdalanae[119]. Plants of Agave vilmorinianaresponded positively to CO 2 enrichment
only when water supply during growth was limited [120,121]. Elevated CO 2 did not enhance either
40 VU ET AL.