Agriculture Reference
In-Depth Information
photosynthetic acclimation to CO
2
enrichment in rice plants with excessive N
supply, but not in those with lower N supply. It was speculated that C and N
reduction compete for electrons from the photosynthetic light reaction, and that this
leads to an inhibition in nitrate assimilation. Bloom et al. (
2010
) also suggested that
the inhibition of nitrate assimilation by elevated [CO
2
] might affect nitrate uptake
rates as a follow-on effect. If impaired nitrate reduction is a reason for declining
tissue N concentrations then meeting N needs through ammonium NH
4
+
could
alleviate the problem. It has been suggested that ammonium-based fertilisers or the
use of nitrification inhibitors (to avoid conversion of ammonia into nitrate in the
soil) could be considered, provided NH
4
+
toxicity and detrimental effects on soils
can be managed (Bloom
2009
). A recent study seems to corroborate this hypothesis
because plants supplied with NH
4
+
as an N source showed better responses to
elevated [CO
2
] than those supplied with NO
3
(Carlisle et al.
2012
). However, this
approach has not yet been tested under realistic field conditions. A similar principle
could also apply to S reduction, but whether S reduction rates are affected by
elevated [CO
2
] has not been tested.
Remobilisation of Nutrients
The remobilisation patterns of nutrients from leaves to developing seeds have partic-
ular significance for grains of cereal crops (Gregersen
2011
). For example, Palta and
Fillery (
1993
) investigated N remobilisation patterns in wheat and they found that N
acquisition from the soil stopped by anthesis and that ~69 % of the N allocated to the
spike was derived from remobilisation from the vegetative plant parts. Remobilisation
strategies are particularly relevant for low rainfall cropping systems where crops are
water-limited during the grain filling period and therefore rely heavily on the nutrients
already stored in vegetative plant parts pre-anthesis, with flag leaves representing the
strongest source of nutrients (Palta and Fillery
1993
). Nutrient remobilisation is
closely linked to leaf senescence processes and for optimum remobilisation exact
timing of processes is crucial (Gregersen
2011
; Yang and Zhang
2006
).
Nutrient and carbohydrate remobilisation patterns are already of interest in the
quest for more nutrient efficient crops (Gregersen
2011
), but little is known about
whether changes in remobilisation patterns contribute to the decrease in nutrient
concentrations, particularly in grains, under CO
2
enrichment. In a study on barley it
was proposed that developing grains have a greater sink capacity for N, as they
receive ample supplies of carbohydrates. N reserves in the leaves may become
depleted faster, leading to accelerated senescence of flag leaves (Fangmeier
et al.
2000
). One recent study on canola in a high rainfall system found that elevated
[CO
2
] significantly decreased N remobilisation to seeds (Franzaring et al.
2012
):
The NHI (see definition in Table
9.1
) decreased by as much as 65 % under ample N
supply. It seems that changes in nutrient remobilisation can be significant, but field
studies with elevated [CO
2
] under soil drying conditions, for example in low
rainfall systems, where the remobilisation of nutrients is relatively most important
(Palta et al.
1994
; Yang et al.
2000
), are not yet available.
