Agriculture Reference
In-Depth Information
highlighting the importance to establish FACE experiments in major and
representative cropping areas (Amthor
2001
; Tausz et al.
2013
). There are a number
of large-scale (10-20 m diameter plots) FACE facilities in agricultural systems
currently in operation globally (Tausz et al.
2013
).
Plant Metabolism Changes Under Elevated [CO
2
]
CO
2
enrichment affects C3 plants primarily through increases in photosynthesis
rate (A) and a reduction in stomatal conductance (g
s
); all other effects of CO
2
enrichment on plant metabolism and growth are linked to changes in these pro-
cesses (Fig.
9.1
; Ainsworth and Rogers
2007
). Increases in A under CO
2
enrichment
occur because of particular properties of the key carbon fixation reaction catalysed
by Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Firstly, Rubisco
has a low affinity for CO
2
as a substrate for carboxylation, which means that the
reaction is not saturated at current atmospheric [CO
2
]. Consequently, a rise in
[CO
2
] increases the carboxylation rate in this reaction, resulting in greater A
(Drake et al.
1997
). Secondly, Rubisco also catalyses the oxygenation of
Ribulose-1,5-bisphosphate (RubP) within the photorespiratory pathway. Photores-
piration is competitively inhibited by CO
2
and increases in atmospheric [CO
2
] will
therefore suppress this pathway (Moore et al.
1999
).
In contrast to photosynthesis, the mechanisms responsible for the reduction of g
s
under CO
2
enrichment remain vague. Ainsworth and Rogers (
2007
) conducted a
meta-analysis of stomatal density responses to CO
2
enrichment and they found that
an average 5 % decrease in density was not statistically significant. They concluded
that rather than changes in stomatal density, changes in stomatal aperture are
responsible for decreased g
s
under high [CO
2
]. Stomatal aperture is determined
Yield
Biomass
Resource Use Efficiency
Photosynthesis
CO
2
Stomatal conductance
Nutrients
Proteins
Elevated CO
2
Ambient CO
2
Fig. 9.1 General responses of crops grown under elevated [CO
2
]. With rising [CO
2
] grain yield,
biomass, photosynthesis, nutrient and water use efficiency increase while stomatal conductance,
nutrient concentrations and protein concentrations decrease. The picture on the
right
shows wheat
(
Triticum aestivum
L.) grown within the Australian Grains Free Air Carbon dioxide Enrichment
(
AGFACE
) facility in Horsham, Victoria, Australia, either under an elevated CO
2
concentration of
~550
mol mol
1
(on the
left
) or under an ambient CO
2
concentration of ~395
mol mol
1
(on the
right
). Elevated [CO
2
] grown wheat shows a significant increase in tiller number and therefore in
μ
μ
