Agriculture Reference
In-Depth Information
Fig. 1.3 The complexity of NUE - In an agronomical context the plant is regarded as an input-
output system with an inherent efficiency that shall be improved (a). Numerous physiological
processes are determining the NUE of a plant (b) and the actual contribution of each process to the
NUE of the plant is biased by many factors of the three variables plant, environment and nutrient
(c). To develop strategies for the improvement of NUE in an agricultural system both the
physiological processes involved and the factors that influence their contribution to NUE have
to be considered
improvement for all species (and even cultivars). To identify the physiological
processes whose modification could increase the NUE of a respective crop, the
particular characteristics should be carefully taken into account and adequate
comparisons of NUE are often only possible within cultivars or strains of the
same species. However, the repertoire of physiological strategies in nature can
also serve as a pool of mechanistic possibilities to improve NUE,
e.g.
by transfer-
ring beneficial traits within distantly related species via transgenic methods.
Another plant-specific variable is the kind of yield the crop will produce. As
described in the introduction the
intended outcome
in agriculture is a maximum
quantity of yield. However, the specific NUE and the way to improve it will differ
fundamentally depending on the desired yield
quality
. The relevance of the phys-
iological processes described above for NUE shifts completely if the desired yield
is starch or sugar and not proteins or oil. While for the former efficient biomass
production and C storage will be important, for the latter allocation to the seeds
increases in relevance. Furthermore, virtually all morphological parts can be the
yield organ into which the desired compounds are allocated before it is finally
harvested.
Environment is the second variable that has an important impact on NUE.
According to Evans and Fischer (
1999
) yield potential (Yp) can be defined as
'
the yield of a cultivar when grown in environments to which it is adapted, with
nutrients and water non-limiting, and with pests, diseases, weeds, lodging and other
stresses effectively controlled
. The yield of a crop depends largely on the envi-
ronmental conditions during the growing period. Nutrients are not always the
limiting factor for plant growth and crop yield. Environmental factors such as
temperature, light and rain or soil-specific factors such as soil composition, pH or
pollution with salts or heavy metals may also be of great significance. If this is the
case, there are more urgent steps to be taken to increase productivity than increasing
NUE (Boyer
1982
). Even if nutrient availability is the limiting factor,
'
the
