Agriculture Reference
In-Depth Information
GSLs currently identifi ed are divided into three groups comprising aromatic, aliphatic
or indolyl side chains, which infl uence the type and biological activity of the various
hydrolysis products. All GSL-containing plants also produce a hydrolytic enzyme (thio-
glucosidase hydrolase) commonly known as myrosinase, which is physically separated
from the GSLs in the intact plant tissue (Rosa
et al
., 1997). On tissue disruption the
myrosinase hydrolyzes the GSLs to form a number of hydrolysis products, a dynamic
evolutionary link that has led to the term 'glucosinolate-myrosinase system' (Bones &
Rossiter, 1996; Rask
et al
., 2000) (Figure 9.1). The system is thought to have evolved in
plants as a defense against generalist herbivores although complex biological interactions
between specialist pests and pathogens have since emerged (Benderoth
et al
., 2006).
GSLs themselves have limited biological activity, but the various hydrolysis products
are responsible for the biofumigant properties as well as the fl avor, anti-nutritional and
therapeutic characteristics of
Brassica
vegetables and spices (Fahey
et al
., 2001; Holst
& Williamson, 2004).
The biologically active hydrolysis products include ITCs, organic cyanides,
oxazolidinethiones and ionic thiocyanates (Brown & Morra, 1997). Among the degrada-
tion products, most focus related to disease control has centered on the ITCs, which are
liberated from aliphatic and aromatic GSLs (Mithen, 2001) (Figure 9.1). They have been
shown to be the most bioactive of the hydrolysis products, and have been recognized
since early in the twentieth century as broad-spectrum biocides (Walker
et al
., 1937).
The use of the synthetic compound methyl ITC (metham sodium) as a soil fumigant
replacement for methyl bromide also generated interest in the idea of utilizing ITCs of
natural origin for disease control (Matthiessen & Kirkegaard, 2006). The variation in the
side-chain structure [R] of some ITCs commonly released from plant tissues is shown in
Figure 9.1.
9.3
Biofumigation can involve GSL-containing plants as rotation crops, or intercrops
(Kirkegaard
et al
., 2000), by incorporating fresh plant material as green manure
(Matthiessen & Kirkegaard, 2006), or utilizing processed plant products high in GSLs
such as seed meals (Borek
et al
., 1997), or dried plant material treated to preserve ITC
activity (Lazzeri
et al
., 2004). Formulations of extracted pure compounds as by-products
of oilseed extraction have also been developed (Palmieri, 2004) and while there may
be a niche for such products, they are likely to be regarded as pesticides by regulatory
authorities and may face signifi cant hurdles in implementation compared with utilization
of rotation crops and green manures (Askew, 2004). The application of high concentra-
tions of specifi c compounds whether 'natural' or synthetic, can bring with it all of the
ecological problems that IPM strategies seek to avoid. Biofumigation implementation
processes all share common features related to the particular chemistry and behavior of
GSL-hydrolysis products in soil discussed later in this chapter. The central focus in this
Chapter is on the utilization of GSL-containing plants as incorporated green manures,
but a brief consideration of the specifi c aspects of the other modes of implementation is
warranted.
Modes of utilization
