Geoscience Reference
In-Depth Information
efficiencies and reduce transpiration by closing stomata through
increased stomatal resistance (Rogers et al., 1980, 1981, 1983,
1984; Kimball and Idso, 1983; Acock, 1990; Goudriaan and
Unsworth, 1990; Parry et al., 1990; Rosenberg et al., 1990;
Stockle et al., 1992a,b; Field et al., 1995; Grant et al., 1999;
Norby et al., 2001; Tubiello and Ewert, 2002; De Costa et al.,
2003; Widodo et al., 2003; Ewert, 2004; Parry et al., 2004;
Ainsworth and Long, 2005). The present levels of CO
2
are sub-
optimal for photosynthesis, but other major factors influencing
growth such as light, water, temperature and nutrients (Osmond
et al., 1980; Downton et al., 1981). Plants will respond differ-
ently according to their biochemical pathways for photosynthe-
sis. C
3
plants will respond vigorously while C
4
plants will not
respond to elevated CO
2
concentration (Alien, 1979; Kimball,
1983a,b; Morison and Gifford, 1983; Acock and Alien, 1986;
Cure, 1986; Alien et al., 1987; Parry et al., 1990).
Increasing atmospheric CO
2
concentrations to 550 ppm
could increase photosynthesis in C
3
crops by nearly 40% (Long
et al., 2004) and these plants have the capacity to carry out
photosynthesis up to 1000 ppm CO
2
, that is, the CO
2
compen-
sation point is higher than the C
4
plants (Kimball, 1985, 1986;
Wittwer, 1985, 1986). There will be no physiological benefits
with rising CO
2
concentrations in C
4
crops such as maize, mil-
let, sorghum and sugarcane as CO
2
is concentrated to 3-6 times
the atmospheric concentrations (von Caemmerer and Furbank,
2003). These crops at such conditions will receive the required
CO
2
in less time, opening the stomata for a shorter duration,
making them more water-use efficient. This may marginally
increase their yields (Long et al., 2004). Crop yield increase
is lower than the photosynthetic response; increase in atmo-
spheric CO
2
to 550 ppm would on average increase C
3
crop
yields by 10-20% and C
4
crop yields by 0-10% (Gifford, 2004;
Long et al., 2004; Ainsworth and Long, 2005). Plants of C
3
photosynthetic pathway may benefit in dry matter production
from an increase in atmospheric concentration of CO
2
through
enhancement of leaf expansion, an increase in photosynthetic
rate per unit leaf area and an increase in water-use efficiency
(Wittwer, 1990). However, different responses of photosynthe-
sis and RuBisco will be encountered among C
3
plant species as
a result of future increases in CO
2
and temperature (Vu et al.,
1997).
The other beneficial effects are usually increases in leaf
area and thickness, stem height, branching, seed and fruit
number and weight, C:N ratio, organ size with higher root-to-
shoot ratios, harvest index or yields of marketable products
