Agriculture Reference
In-Depth Information
Expression of NP as unit biomass produced per unit nutrient may not always be
the most suitable measure. The desired product in an agricultural system is not
always biomass, consisting of structural or non-structural carbohydrates, but more
often seeds that are rich in proteins or oil. It is thereby not only important how a
nutrient contributes to growth but also how it improves the yield and quality of the
desired product. Therefore, the respective nutrient can itself be a substrate for the
production (
e.g.
as N and sulfur (S) for proteins) or a facilitator of the production
(
e.g.
by being a component of an enzyme involved). Consequently NP on the basis
of biomass may not always be the best measure and a more general indicator may be
yield productivity (YP), which includes quantity and quality of the desired yield
product per unit nutrient in the plant tissue. However, in agriculture and in general
for all nutrients, NP (or YP) and MRT can be seen as sub-components of NUtE.
Physiological Processes Involved in NUE
After the derivation of a conceptual framework the key physiological processes
involved in the complex trait NUE will be briefly summarized (Fig.
1.2
). As
described above, NAcE is one main component of NUE and consequently nutrient
uptake is one of the key processes involved. Although some nutrients can be
derived from the atmosphere (
viz
. N and S; Faller
1972
; Stulen et al.
1998
;De
Kok et al.
2007
), the plant largely depends on mineral nutrients taken up from the
soil (Mengel and Kirkby
1987
; Marschner
2012
). These are either derived from
weathering of parental rock material or biological breakdown of organic matter and
the chemical availability to the plant depends on soil-specific properties which in
turn determine the proportion of nutrients dissolved in the soil water (usually less
than 0.2 %), bound to organic detritus (around 98 %) or adsorbed by soil colloids
(Larcher
1995
).
There has been much discussion on the significance of NAcE in explaining
differences in NUE between plants. Most studies were focussed exclusively on N
and came to different conclusions. For corn (
Zea mays L
.) it has been concluded
from a study with different hybrids that NAcE is only relevant for differences in
NitUE if the outside N concentrations are high, while NUtE of accumulated N was
the driving variable if the supply was low (Moll et al.
1982
). Whereas in pumpkin
NAcE was not a possible target to improve NUE at either high or low N concen-
trations (Swiader et al.
1994
). However, recent studies have suggested that an
increased acidification capacity of the rhizosphere could be targeted to increase
nitrate uptake and improve NUE (Paez-Valencia et al.
2013
). In addition, the NUE
of an agricultural system may be improved if plants could maintain internal nutrient
concentrations and optimal growth with a lower outside concentration in the soil.
Therefore understanding the response mechanisms of NAcE to nutrient deficiencies
may improve the ability of crops to tolerate lower nutrient concentrations in the soil
and thereby save fertiliser and reduce potential pollution.
