II.3. BRASSICA CROPS (BRASSICA SPP.) – 229
The glucosinolates serve as an advance-prepared system of protection that is activated
only when plant tissue is damaged by a disease or insect attack. The destruction of the
plant cells results in the hydrolysis of the glucosinolates by the myrosinase enzyme, thus
releasing the volatile isothiocyanates that have a wide spectrum of anti-microbial effects
and act as attractants or repellents to some insects and herbivores (Vašák, 2002; Brown
and Morra, 1997; Fenwick et al., 1983).
Brassica crops are also used as biofumigants based on the release of the bioactive
isothiocyanates in the soil when seed meal amendments or green manure are
incorporated, or Brassica crops are used in the rotation (Brown and Morra, 1997).
It is also suspected that the volatile isothiocyanates, from residue of Brassica crops, result
in inhibitory effects on some subsequent crops (see the section on “Allelopathy”).
The glucosinolates also impact on the health and nutrition of animals and humans as
well as the quality and usefulness of products from Brassica crops. These aspects are
discussed in the following section.
Figure 3.40. Glucosinolate chemical structure and enzymatic breakdown products formed in
broken Brassica plant cells with moistureDamaging insects
Brassica species are important components of temperate climate ecosystems.
They provide forage for many insects as well as wild life. The complex of insects that
feed upon the Brassicas is one of the important factors limiting the production of
commercial Brassica crops (Ekbom, 1995; Lamb, 1989). Brassicaceous plants produce a
family of sulphur compounds called glucosinolates, whose breakdown products are
attractants and stimuli for feeding and oviposition but, on the other hand, act as deterrents
or toxins for herbivores not adapted to plants of the Brassicaceae. A list of insects
important to Brassica plants is given in Table 3.A1.1 of the annex. Some of the more
important insects are discussed below.