GLUCOSINOLATE RESPONSES OF SWEDE, KALE, FORAGE AND OILSEED RAPE TO ROOT DAMAGE BY TURNIP ROOT FLY (DELIA-FLORALIS) LARVAE

Two genotypes of kale (Brassica oleracea L), two of rape (one forage and one winter oilseed type, B napus L) and two of swede (B napus L) grown in a glasshouse were inoculated with turnip root fly (Delia floralis Fall) eggs at 5 weeks and their roots, stems and leaves sampled after a further 8 weeks of larval feeding. Changes in total and individual glucosinolates were examined in roots, stems and leaves after attack. Damage by D floralis larvae to roots caused tissue weight losses in roots ranging from 26 to 48 %, relative to controls. The resultant plant stress also caused smaller weight losses in leaf tissue (10-27 %) and stems (6-20 %). On susceptible brassica genotypes, D floralis percent pupal development and mean pupal weights were a factor of x 1.5 greater than on resistant genotypes, resulting in a difference of x 2.4 in total pupal biomass per root. There was no apparent link between D floralis susceptibility and total or individual glucosinolate content of undamaged control roots. Major differences in glucosinolate composition were found between control roots of the six brassicas. Five of the 15 compounds detected had aromatic side chains and were present in all samples. Only three of the 10 aliphatic glucosinolates were common to all six brassicas. The predominant compound in the two kale roots was prop-2-enyl glucosinolate, in the rapes 2-hydroxy-but-3-enyl (forage rape cv Hobson) and 2-phenylethyl (oilseed rape cv Ariana) glucosinolates, and in the swedes 2-hydroxy-but-3-enyl glucosinolate. All brassicas showed a similar response in glucosinolate metabolism after root damage. Total glucosinolate content in roots increased due to a two to four fold increase in indole-based compounds. The largest increase for an individual compound after attack was found for 1-methoxy-3-indolylmethyl glucosinolate, which increased four to 17 fold. Root damage did not significantly affect stem glucosinolate composition but in leaves caused a 1.2 fold increase in aliphatic glucosinolates with a corresponding decrease in indole-based compounds. The possible ecological and nutritional significance of glucosinolate responses to root fly damage is discussed.