Agriculture Reference
In-Depth Information
by the turgor pressure in guard cells via ion and organic solute concentrations
(Messinger et al.
2006
). Rising [CO
2
] may be linked to depolarising the membrane
potential in guard cells, resulting in stomatal closure (Ainsworth and Rogers
2007
;
Drake et al.
1997
; Messinger et al.
2006
).
A meta-analysis on the responses of A and g
s
to CO
2
enrichment under FACE
conditions confirmed that in C3 crops light-saturated CO
2
uptake rates (A
sat
)
increased on average by ~13 % while stomatal conductance is reduced by ~25 %
(Ainsworth and Rogers
2007
). Increases in A have stimulating effects on plant
growth and productivity (Fig.
9.1
; Ziska et al.
2012
) and, depending on the plant
species investigated, greater A leads to greater biomass accumulation and yield
(termed “CO
2
fertilisation effect”). For example, C3 crops respond to high [CO
2
]
with ~20 % greater biomass and ~17 % greater yield (Ainsworth and Rogers
2007
;
Tausz-Posch et al.
2012
). It is noteworthy that in the same meta-analysis (Ains-
worth and Rogers
2007
) C4 crops also showed a significant ~11 % increase in A
sat
in response to CO
2
enrichment. In general, C4 crops are more independent of
ambient [CO
2
] as they have the ability to concentrate CO
2
in the leaf mesophyll.
Their carboxylation reaction is generally close to saturated under current ambient
[CO
2
]. The positive response of C4 plants to CO
2
enrichment was linked to the
improved water status of the crops investigated (Ainsworth and Rogers
2007
).
The initial stimulation of C3 photosynthesis may not be sustained when plants
are exposed to CO
2
enrichment for long time periods (Kirschbaum
2011
; Moore
et al.
1999
). Plants grown under CO
2
enrichment showed decreased A relative to
plants grown under ambient [CO
2
] when both were measured at ambient
[CO
2
]. This down-regulation of photosynthesis under CO
2
enrichment is called
“photosynthetic acclimation” (Moore et al.
1999
). Photosynthetic acclimation is
linked to reduced concentrations of Rubisco in the leaves, and this is hypothesised
to result from repression of Rubisco synthesis by accumulating carbohydrates
(Drake et al.
1997
). If there are insufficient sinks for the increased amounts of
carbohydrates produced in photosynthesis, this situation is accentuated (Moore
et al.
1999
; Stitt and Krapp
1999
). Although such acclimation reduces the photo-
synthetic capacity of plants, it does normally not completely offset the stimulation
caused by CO
2
enrichment (Ainsworth and Rogers
2007
).
Mineral Nutrition of Crops Under Elevated [CO
2
]
With changes in growth and yield, plants grown under CO
2
enrichment also show
significant changes in their chemical composition (Erbs et al.
2010
). Loladze
reported in 2002 that CO
2
enrichment will alter the stoichiometry (relative elemen-
tal composition) of plants leading to reduced concentrations of most macro and
micro elements in plant tissues. As a consequence there are potential negative
consequences for the nutritional value of crops (Loladze
2002
). For example,
decreased tissue nitrogen (N) concentrations under elevated [CO
2
] have been
widely reported (Stitt and Krapp
1999
). As a large proportion of N in plants is
used for protein synthesis, protein concentrations in vegetative parts as well as
