Agriculture Reference
In-Depth Information
(Fig.
8.5a
). In particular, later responses all channel into a general senescence
programme comparable to developmental senescence (Fig.
8.5b
; Watanabe
et al.
2010
,
2012
,
2013
).
Nutrient ion availability in the field depends on physical parameters such as soil
type and soil chemistry, weather conditions, leaching and run-off and on agricul-
tural parameters such as the amount of minerals removed from the field with the
harvest and fertilisation to replenish resources (Hawkesford
2012
). Fertilisation is
usually employed to counteract deficits, though this becomes increasingly costly
due to rising energy costs,
e.g.
for nitrogen production, and the fact that some
minerals, such as phosphate, might become limiting in the future which would
likely increase prices (Gamuyao et al.
2012
). Furthermore, fertilisation carries the
risk of polluting the environment, and it is therefore necessary to improve applica-
tion procedures and to breed mineral nutrient-efficient crop cultivars (McAllister
et al.
2012
). In order to strategically support breeding efforts, it is necessary to
understand the processes and identify the genes affecting nutrient use efficiency.
However, the situation in a field can be very complex as nutrients can be available
in varying amounts and combinations. Deficiency might exist from germination on
poor soils or might become established during growth due to active depletion of soil
minerals by the plants or by environmental conditions such as heavy rainfalls.
Sufficient minerals might even be present in a given soil but not accessible to the
plant due to unfavourable soil pH, structure or drought. The effect of distinct
mineral nutrient stresses on plants is well documented from agricultural practice
and from long-term, systematic field experiments, such as the Broadbalk field trial
at Rothamsted Research, UK (Lu et al.
2005
; Shinmachi et al.
2010
). Plants react to
mineral nutrient depletion with gross phenotypic responses (Fig.
8.3a
), such as
reduction of biomass and seed yield, alteration of leaf pigment contents and
distribution, and alteration of root morphology (Lopez-Bucio et al.
2006
; Gruber
et al.
2013
, Nikiforova et al.
2004
; Walch-Liu et al.
2006
; Marschner
2012
). It is
suggested that the plant response to nutrient deprivation can be divided into two
major phases (Fig.
8.3b
).
During the initial rescue phase, the plant tries to counteract mineral deficien-
cies by inducing reversible adaptation mechanisms including increased uptake in
the root zone, mobilisation of internal resources, and by preventing biosynthetic
investments dependent on the depleted nutrient. High-affinity transporters get
induced in roots, which allow the plant to accumulate ions against steep con-
centration gradients present between soil with very low ion concentrations and
root tissue. Lateral root development is decreased in favour of exploratory growth
of the main roots (Gruber et al.
2013
; Hubberten et al.
2009
,
2012b
;Drewand
Saker
1975
; Walch-Liu et al.
2006
), assimilatory enzymes are activated and the
respective genes are induced (Hoefgen and Hesse
2008
; Davidian and Kopriv
2010
). Additionally, previously deposited internal stores of limiting elements
such as those found in the vacuole are utilised by inducing vacuolar transporters
in the tonoplast. In the face of continued deficiency, these primary responses are
supported by strategies promoting degradation processes or by delaying anabolic
processes. Even mild nitrogen starvation quickly affects polysome loading and
