Agriculture Reference
In-Depth Information
(White and Brown
2010
). However, this practice needs to be used optimally in
agriculture as too high concentration of some of these elements in the soil solution
may inhibit plant growth and reduce crop yield (White and Brown
2010
).
Adequate sulfur supply is required for proper growth and fitness of all living
organisms. Sulfur is present in a wide variety of metabolites with specific biological
functions. Sulfur is cycled in the global ecosystem and can be converted to its
organic compounds by photosynthetic organisms and microorganisms. The most
common form of sulfur in nature is sulfate (SO
4
2
), which is the most oxidised
form, in which sulfur is in the +VI redox state. Sulfate is taken up from the soil and
reduced to sulfide in an energy-dependent reduction pathway. Sulfide can be
incorporated into
O
-acetylserine to form cysteine, which is the first stable form of
bound reduced sulfur in plants. Apart from cysteine, sulfur is present in methionine,
sulfolipids, and cell walls. It also is contained in thiols, which are involved in redox
control in plant cells and in vitamins and cofactors such as coenzyme A, thiamine
and biotin. Noteworthy are the sulfur-containing secondary metabolites such as
alliins and glucosinolates. Their breakdown products are responsible for character-
istic smell and taste of many vegetables but also deter plant pathogens. Both of
these classes of natural products are also very beneficial for human health. Alliins,
found in large amounts in garlic, have antimicrobial properties, and glucosinolate
degradation products induce enzymes that prevent tumour formation.
Sulfur availability in agricultural soils has been decreasing in many areas of
Europe during the last two decades (McGrath et al.
1996
; Zhao et al.
1996
). The
main reasons are: (i) reduction of sulfur dioxide emissions and subsequent reduc-
tion in sulfur depositions and (ii) changes in fertiliser practice, i.e. higher definition
fertilisers without sulfate contamination (Blake-Kalff et al.
2001
). Additionally, it
was suggested that the demand for sulfur in many crops has increased due to
intensive agriculture and optimisation during plant breeding programmes (Abdallah
et al.
2010
). The crop
s requirement for sulfur varies in different species. Generally,
wheat requires approximately 2-3 kg of sulfur for each tonne of grain produced
(Zhao et al.
1999
) whereas the production of 1 t of oilseed rape seeds requires about
16 kg of sulfur (McGrath and Zhao
1996
). This high demand for sulfur of oilseed
rape is probably due to an ineffective xylem-to-phloem transport mechanism in this
species. A large amount of sulfate is accumulated in the vacuoles of mature oilseed
rape leaves in sulfur sufficient conditions. This sulfate is not easily available for
redistribution if sulfur is limited (Blake-Kalff et al.
1998
). Therefore oilseed rape is
particularly sensitive to sulfur deficiency or limitation which reduce both seed
quality and yield (Malhi et al.
2007
). Sulfur deficiency in crop plants has been
recognised as a limiting factor not only for crop growth and seed yield but also for
poor quality of products which is particularly important in the case of wheat and the
maintenance of baking quality (Shahsavani and Gholami
2008
). The protein frac-
tion is known to play an essential role in the bread-making quality of wheat. The
gluten proteins, gliadins and glutenins, represent about 80-85 % of total flour
protein. These are responsible for elasticity and extensibility that are essential for
functionality of wheat flours (Hussain et al.
2012
; Kuktaite et al.
2004
). However, in
sulfur-limited conditions the synthesis of sulfur-poor storage proteins such as
'
