Agriculture Reference
In-Depth Information
160
140
120
100
80
60
40
20
0
Oct6
Oct28
Nov19
Dec12
Fig. 9.2 Total free amino acid concentrations in wheat flag leaves from heading (Oct 6) to
maturity (Dec 12). Plants were grown either under ambient CO
2
(
closed symbols
) or an elevated
CO
2
concentration of ~550 ppm (
open symbols
) within the AGFACE facility, Horsham, Australia.
Values are means (standard errors) with four replications in each
For wheat, a decrease in leaf N was also related to decreases in leaf chlorophyll
across a number of experiments (by 8 % according to a synthesis by Wang
et al.
2013
), but such changes in chlorophyll are not consistently found. High
[CO
2
] can also lead to decreases of leaf nitrate concentrations (e.g. for wheat reported
in Hocking and Meyer
1991
). There is little information on free amino acids, but
recent FACE data indicate about 20-30 % decreases in wheat leaves (Fig.
9.2
).
There is quite a range in responses between different experiments, environments
or species. Most consistent among such variability is that decreases in leaf N
m
are
less in legumes than in non-N fixing species, perhaps because legumes can channel
excess carbohydrates towards greater nitrogen fixation rates (Rogers et al.
2009
).
Decreases in grain protein contents in the final harvest are commonly
unfavourable for food or feed quality, and of particular concern in cereals such as
wheat, where bread making and marketing quality is largely determined by protein
content (Loladze
2002
; Taub et al.
2008
). Grain N decreased under CO
2
enrichment
by between 0 % and 20 % in cereals (wheat, rice, barley; as reviewed by H¨gy and
Fangmeier
2008
;H
¨
gy et al.
2013
; Taub et al.
2008
) and by less than 5 % (but still
significantly so) in soybean (Taub et al.
2008
).
Can Greater N Supply Restore Leaf and Grain N?
It is uncertain to what extent N supply determines the extent to which N
m
decreases
in leaves or grains under CO
2
enrichment: Some evidence suggested that N
m
decreases were most pronounced in N-deficient and less marked or even absent in
well fertilised plants (Stitt and Krapp
1999
), including in FACE experiments
