PHENOLIC-ACIDS ASSOCIATED WITH THE RESISTANCE OF LETTUCE CULTIVARS TO THE LETTUCE ROOT APHID

Phenolic acids, particularly caffeoylquinic acids, in lettuce root extracts were analyzed by high pressure liquid chromatography (hplc), gas liquid chromatography (glc) and UV absorbance to seek a relationship with the resistance of certain lettuce cultivars to lettuce root aphid, P. bursarius. Although consistent results were obtained by each method, glc estimates of isochlorogenic acid tended to be low and UV estimates high. Hplc results were intermediate and since it was the easiest technique to perform routinely it was preferred. Isochlorogenic acid was the only caffeoylquinic acid detected in quantity and there were greater concentrations in resistant than in susceptible cultivars. Phenylalanine-ammonia lyase (PAL), the 1st enzyme in the phenyl propanoid pathway, is more active in resistant cultivars. Although these cultivars also had a greater tendency to browning when damaged, it was not due to greater polyphenol oxidase activity but probably to the presence of more isochlorogenic acid substrate. The results were consistent with an association between isochlorogenic acid concentration, PAL activity and resistance to P. bursarius.