Agriculture Reference
In-Depth Information
Sulfur deficiency may not be easily recognised in the field as the symptoms are
not obvious, except in severely deficient plants. In general, sulfur deficiency results
in uniform pale green chlorosis through the plant. A considerable reduction in
growth may be suffered without the appearance of any other visible symptoms.
Clear symptoms are associated with severe stunting, reduced leaf size and activity
of axillary buds, which results in less branching. Physiologically in wheat plants,
sulfur deficiency affects CO
2
assimilation rates and Rubisco enzyme activity as
well as protein abundance which results in general inhibition of
de novo
synthesis
of the photosynthetic apparatus (Gilbert et al.
1997
). Additionally, depression of
root hydraulic conductivity was observed in sulfur-deficient barley plants. It was
suggested that this response can have a role in signalling nutrient starvation from
shoots to roots (Karmoker et al.
1991
). Sulfur deficiency is dependent on the soil
type and predominant climatic conditions and does not occur uniformly. Therefore,
a reliable field based test is required to determine where it is likely to occur (Blake-
Kalff et al.
2000
). Soil testing can provide information about the amount of sulfur
available to plants. However, there is no direct correlation between the content of
sulfate in the soil solution and plant yield in field conditions (Zhao and McGrath
1994
). Thus, the analysis of sulfur content in plant tissue might be a better indicator
of sulfur available for the plant at the time of sampling. However the determination
of a reliable diagnostic indicator is problematic because of constantly changing
environmental conditions, variation in sulfur metabolism between species and plant
demands (Blake-Kalff et al.
2000
).
Genetically engineered plants are one of the most promising approaches to
achieve an enhancement of sulfate use efficiency. Possible targets for genetic
engineering might be focused on improved resource capture or efficient utilisation
of increased uptake (Hawkesford
2000
). The first goal could be achieved by
modulation of transport systems. It is known that the expression of sulfate trans-
porters is controlled by sulfur availability. During sulfur limitation the expression
of sulfate transporter genes increases significantly, but it decreases when sulfate is
abundant. Overriding this control might be achieved by expressing transporter
genes under the control of an appropriate constitutive promoter. However, the
control mechanisms would have to be removed only for sulfate transport and not
for other steps of the pathway in order to prevent for example accumulation of
sulfide, which is toxic for plants in high concentration. Alternatively, root structure
and proliferation could be targeted. The second goal might be achieved by improv-
ing the mobilisation of vacuolar reserves or introducing an increased demand for
sulfate to stimulate further sulfate uptake (Hawkesford
2000
). In order to find the
most efficient targets, further exploration of sulfate metabolism pathway and the
regulatory mechanisms is required.
