Twenty eight Brassica napus lines were developed which had contrasting leaf glucosinolate profiles to those found in commercial oilseed rape cultivars. The lines varied both in the total amount of aliphatic glucosinolates and in the ratio of different side chain structures. The lines were used in field experiments to assess the manner by which glucosinolates mediate the interactions between Brasssica and specialist pests (Psylliodes chrysocephala and Pieris rapae) and generalist pests (pigeons and slugs). Increases in the level of glucosinolates resulted in greater damage by adult flea beetles ( P . chrysocephala) and a greater incidence of Pieris rapae larvae, but reduced the extent of grazing by pigeons and slugs. Decreasing the side chain length of aliphatic glucosinolates and reducing the extent of hydroxylation of butenyl glucosinolates increased the extent of adult flea beetle feeding. The implications of modifying the glucosinolate content of the leaves of oilseed rape and the role of these secondary metabolites in plant/herbivore interactions are discussed.

Significant differences occur in the levels and types of aliphatic glucosinolates in leaves of plants of four Brassica oleracea populations in Dorset. Plants in grassland at St Aldhelm's Head and Winspit have high levels of 3-butenyl glucosinolate, whereas plants of an adjacent population growing on and along the top of cliffs at Kimmeridge have low levels of 2-hydroxy-3-butenyl, 2-propenyl and methylsuiphinylalkyl glucosinolates. Plants growing in a variable habitat at Worbarrow Tout have intermediate levels. The differences in occurrence of individual glucosinolates result from allelic variation at four loci. The level of total aliphatic glucosinolates is under more complex genetic control, but is shown to be highly heritable. Allele frequencies at isozyme loci indicate that genetic variation for glucosinolate production is unlikely to have arisen or to be maintained by founder effects or genetic drift. It is suggested that there is selection for high levels of butenyl glucosinolates at St Aldhelm's Head and Winspit because of grazing by generalist herbivores, whereas there is selection for low levels of 2-hydroxy-3-butenyl and other non-butenyl aliphatic glucosinolates at Kimmeridge because of two factors. First, plants effectively escape from generalist herbivores because of physical aspects of the habitat and association with other plant species which provide physical and chemical defences. Thus there is selection for individuals which do not carry the hypothetical metabolic costs of glucosinolate biosynthesis. Secondly, herbivory by specialist cruciferous insects at Kimmeridge, which is enhanced because of the local abundance of B. nigra, selects for individuals which have low levels of 2-hydroxy-3-butenyl glucosinolates.

The biochemical and genetical relationship between aliphatic glucosinolates which have methylthioalkyl, methylsulphinylalkyl and alkenyl side chains has not been resolved by biochemical studies. In this study, two hypothetical models are tested by the genetic analysis of a backcross population between Brassica drepanensis and B. atlantica. The results support one of the models in which 3-methylthiopropyl glucosinolate is sequentially converted to 3-methylsulphinylpropyl, and then to 2-propenyl glucosinolate, by the action of dominant alleles at two loci. RFLP mapping positioned both loci on the same linkage group homologous to the B. napus N19 linkage group. The implication of the results for the genetic manipulation of glucosinolates in Brassica to improve flavour and nutritional properties, and in order to investigate plant-insect interactions, is discussed.

The inheritance of aliphatic glucosinolates was studied in crosses between synthetic B. napus lines and oilseed rape cultivars. Six unlinked loci are described which determine the aliphatic glucosinolate profile of B. napus. One locus regulates the presence or absence of propyl glucosinolates, while another regulates the expression of pentyl glucosinolates. Two loci regulate the removal of the terminal H3CSgroup from the amino acid derivative to produce alkenyl glucosinolates as opposed to methylthioalkyl and methylsulphinylalkyl glucosinolates, regardless of the length of the alkyl chain. Likewise, another two loci regulate the hydroxylation of both butenyl and pentenyl glucosinolates. The functional alleles at one of the hydroxylation loci results in significantly more hydroxylation than those at the other locus. The large number of aliphatic glucosinolates which have been described in Brassica thus results from an interaction between genes which regulate side chain elongation and genes which modify the structure of the side chain, regardless of its length. The implications of this study for the biosynthesis of aliphatic glucosinoiates, the origin of B. napus and the potential to manipulate the leaf and seed glucosinolate profile of oilseed rape are discussed.

The hypothesis that enhancing the level of glucosinolates in leaves of oilseed rape would increase resistance to two fungal pathogens was tested. Thirty-three Brassica napus lines with variable leaf glucosinolate contents were assessed for susceptibility to infection by Leptosphaeria maculans and Alternaria spp. under field conditions. The data clearly showed that there was not a simple positive relation between glucosinolate content and resistance, and that the hypothesis can be refuted. The level of Alternaria infection of both leaves and pods was positively correlated with glucosinolate content, while for L. maculans there was no significant relationship between the two variables. It is suggested that these pathogens have become specialists on glucosinolate-containing taxa in an analogous way to insect herbivores.

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