Agriculture Reference
In-Depth Information
regarded as a constraint
to NUE and a possible target for its improvement
(Hawkesford
2000
).
For biomass production plants convert inorganic carbon dioxide from the atmo-
sphere to organic carbohydrates via photosynthesis. Fuelled by the energy of the
sun, this process is the primary generator of all biomass on earth. Although N is
most directly linked to photosynthesis, all the essential nutrients contribute in some
form to growth promotion
i.e.
biomass production. This can happen directly if the
nutrient is part of the carbon-assimilating apparatus or indirectly if it plays a role in
energy transfer, defence, homeostasis, tolerance and other processes that facilitate
optimal plant functioning. Consequently for every nutrient a respective NP can be
assigned which is a measure for the biomass produced per unit of the nutrient in the
plant. However, the mechanisms underlying this component of NUE and how the
NP (and consequently the NUtE) of a certain nutrient can be improved are manifold
and depend on the specific role that a nutrient plays in plant metabolism.
Again N is studied most intensively, and due to its direct link to photosynthesis
and biomass production, there is a clear cut correlation with the NP of N and (i) the
amount of total N invested in the photosynthetic tissue, (ii) the N efficiency of
photosynthesis and (iii) relatively low loss of carbon due to respiration (
gren
1985
; Poorter et al.
1990
). The carbon-assimilating enzyme Rubisco is currently
one of the most prominent targets for possible genetic improvement of photosyn-
thesis (Loomis and Amthor
1999
; Parry et al.
2011
), largely due to its apparent
catalytic inefficiency in carboxylation and its consequent high abundance. The idea
is that a higher efficiency would lead to less Rubisco being needed to maintain the
same rate of photosynthesis and consequently, as this enzyme contains high
amounts of N, a higher NUE of N. One intriguing approach is the attempt to express
the Rubisco of some non-green algae, which have a greater specificity for CO
2
, into
higher plants (Whitney et al. 2001).
However, is the biochemical inefficiency of photosynthesis really the bottleneck
that hinders higher biomass production and NUE? Although photosynthetic effi-
ciency is in theory one of the key limiting factors for increasing biomass and crop
yields (Long et al.
2006
; Parry et al.
2011
), supportive correlations in practice are
not easy to assess and studies come to different conclusions. Studies on closely
related germplasm of wheat showed a correlation of photosynthetic rate and yield
(Watanabe et al.
1994
), while comparisons of cultivated crops with their wild
ancestors showed that the latter have a higher photosynthetic rate (Evans and
Dunstone
1970
). The potential limiting role of photosynthesis apparently depends
to a greater extent on other processes with negative feedback on photosynthesis. If
the capacity of the sink declines and the flux of photosynthates into sink products
stagnates, this results in a compensating down-regulation of photosynthesis. Con-
sequently the strength of the sink is just as important if not more so for yield as the
efficiency of the source (Zelitch
1982
; Borr
´
s et al.
2004
; Reynolds et al.
2005
).
According to these studies an increased sink capacity is required to increase
photosynthesis and not the other way around. However, field studies with C
3
plant species under exposure to elevated levels of carbon dioxide (eCO
2
) suggested
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