Agriculture Reference
In-Depth Information
synthesised
de novo
and are part of active or induced resistance responses to pathogen
attack. A detailed description of resistance mechanisms in plants can be found elsewhere
(e.g. Toyoda
et al.,
2002; Mayer, 2004).
The S-containing metabolites glutathione and glucosinolates can be classed as
phytoanticipins, while elemental S depositions, pathogenesis-related (PR) proteins such
as plant defensins of class PR-12, thionins of class PR-13 and lipid transfer proteins of
class PR-14 (van Loon
et al.
, 1994), and last but not the least low-molecular-weight anti-
biotics such as camalexin, are part of the active resistance armoury. Van Loon
et al.
(1994)
based his classifi cation of PR proteins on amino acid sequences, serological relationships,
and/or enzymatic and biological activity. So far, it is unclear which group, phytoanticipins
or active resistance responses, the release of H
2
S of plants can be assigned to. A relation-
ship between S supply/S status of the plant and H
2
S emissions in non-infected plants,
which yields fungicidal levels of H
2
S, would point to preformed resistance. An interac-
tion between S supply or S status and non-protein cysteine, and H
2
S emissions of infected
plants as a direct, rapid and effi cient response of the plant to the pathogen, would point to
a mechanism of active or induced resistance.
There are two main prerequisites for an S-containing compound to be involved in
SIR: fi rstly, a signifi cant relationship between plant S status and metabolite content, and
secondly, direct or indirect fungitoxicity of the component.
11.2.2
Relationship between S nutritional status and
S metabolite content
Macroscopic S defi ciency can occur on all soil types and is generally exacerbated by the
following: high yields; soils with light texture, high permeability and low organic matter
content; sites poorly connected to capillary ascending, S-rich groundwater; high rate of
leaching; reduced root growth and rooting intensity; soil compaction or low temperatures
(Haneklaus
et al.
, 2006a). Schnug & Haneklaus (2005) and Haneklaus
et al.
(2006b) pro-
vide a detailed description of the symptomatology of S defi ciency in agricultural crops.
The level of S uptake depends on plant available sulphate in the soil and S fertilisation.
The effi cacy of S fertilisation to increase the S nutritional status depends on the initial S
content (Schnug & Haneklaus, 1994). In the range of severe and moderate S defi ciency
(Haneklaus
et al.
, 2006a), Paulsen (1999), using an S dose of 40 kg ha
−1
applied to oil-
seed rape, obtained an increase in the total S content of younger leaves at the start of stem
elongation, of 58-83 µg g
−1
S per kg S applied. Application of 25 kg ha
−1
S to winter
wheat and winter barley increased the total S content in shoots at stem extension by 52
and 68 µg g
−1
S per kg S, respectively (Paulsen, 1999).
A signifi cant relationship between S fertilisation and GSH, and glucosinolate content,
was found in greenhouse and fi eld experimentation (Haneklaus
et al.
, 2006a). Haneklaus
et al.
(2006a) compared and summarised relevant data from the literature for these metabo-
lites. Under fi eld conditions, the glutathione and glucosinolate content of oilseed rape might
increase by up to 64 and 150 nmol g
−1
dry weight per kg ha
−1
S, respectively (Haneklaus
et al.
, 2006a). S fertilisation increased the free cysteine content in younger leaves of oil-
seed rape by up to 8.7 nmol g
−1
dry weight per kg ha
−1
S applied (Salac, 2005).
First experiments with fi eld-grown oilseed rape indicate that the release of H
2
S is
related to S supply (Table 11.1). The highest peak in the release of H
2
S was found at start
