Studies on populations of the wild cabbage Brassica oleracea L. subsp. oleracea

PhD ThesisIt is established that all British populations of wild cabbage, Brassica oleracea L. subsp. oleracea, show variation with respect to the Guignard picrate test. The picrate response in this species is due primarily to the volatile derivatives of sinigrin : especially allyl-nitrile which is released on damage to laminar tissue. The picrate response varies during the year due to the effects of temperature, but individuals remain distinct. Within a population, there is a relationship between picrate score and age, older plants being mostly of low picrate response. This is due to selection rather than age. The Large White butterfly, Pieris bra~sicae L., can be a major predator of mature, sterile B. oleracea. Gravid female butterflies preferentially select high picrate response plants for oviposition (although the larvae are not selective with respect to picrate response). Larvae will cause considarable damage to host plants, although they are rarely directly responsible for plant death. However, plants which have been heavily predated by the larvae of P. brassicae are susceptible to further attack, e.g. by the aphid Brevicorvyne brassicge L., and are thus further weakened and may finally die. It is suggested that the numbers of high picrate response plants are thus reduced. At the seedling stage the situation is reversed. High picrate response seedlings are at a Selective advantage, the high levels of sinigrin derivatives protecting them from Molluscan and Fungal depredation. Thus selection by P. brassicae on the mature plants is balanced by reverse selection on seedlings. A computer study of twelve morphological characters, demonstrates that high response plants are differentiated from lower response individuals. A historical study strongly suggests that all the British populations are escapes from cultivation.Agricultural Research Council grant (no. AG 13/47)