GLUCOSINOLATE LEVELS IN COTYLEDONS OF MUSTARD, BRASSICA-JUNCEA L AND RAPE, BRASSICA-NAPUS L DO NOT DETERMINE FEEDING RATES OF FLEA BEETLE, PHYLLOTRETA-CRUCIFERAE (GOEZE)

Sinigrin (allyl glucosinolate), the major glucosinolate in the cotyledons of Brassica juncea cv. Cutlass, occurred in the highest concentration and amount at seedling emergence and declined during growth. Glucobrassicin (3-indolylmethyl glucosinolate), the major glucosinolate in the cotyledons of B. napus cv. Westar, occurred in the lowest concentration and amount at seedling emergence. The amount of glucobrassicin per cotyledon pair increased about fourfold during 14 days of growth, but its concentration remained relatively unchanged because of "dilution" by increasing cotyledon biomass. These different glucosinolate profiles indicate a different metabolic control and different biological function for sinigrin and glucobrassicin. The flea beetle, Phyllotreta cruciferae Goeze, does not discriminate between cotyledons having sinigrin or glucobrassicin since the two crucifers were fed upon equally in choice tests. Restricting the concentration of sulfur in the nutrient medium accelerated the decline of sinigrin in B. juncea cv. Cutlass but did not alter the feeding rate of P. cruciferae compared to controls. Sulfur restriction reduced glucobrassicin in B. napus cv. Westar to undetectable levels and somewhat reduced the feeding rate of P. Cruciferae. Nevertheless, P. cruciferae still fed actively on cotyledons of B. napus cv. Westar depleted of glucosinolates and severely damaged many of them. Since glucosinolate type and concentration had little effect on feeding response, reduction or elimination of foliar glucosinolates alone would not seem a useful strategy for protecting seedlings of these two crucifers from flea beetle damage. © 1990 Plenum Publishing Corporation.