PHENYLALANINE AMMONIA-LYASE INHIBITORS DO NOT PREVENT RUSSET SPOTTINGLESION DEVELOPMENT IN LETTUCE MIDRIBS

Russet spotting is a physiological disorder of lettuce (Lactuca sativa L.) caused by exposure to hormonal levels (<1 mu L.L-1) of ethylene i n air at approximate to 5 degrees C. Enhanced phenolic metabolism and the accumulation of phenolic compounds accompany the appearance of bro wn, oval lesions on the leaf midrib. Phenylalanine ammonia-lyase (PAL) is the first committed enzyme in the phenylpropanoid pathway. Three i nhibitors of PAL activity [2-aminoindan-2-phosphonic acid (AIP), alpha -aminooxyacetic acid (AOA), and alpha-aminooxi-beta-phenylpropionic ac id (AOPP)] greatly reduced the accumulation of phenolic compounds and browning of lesions. At a concentration of 50 mu M, AIP inhibited the formation of chlorogenic and dicaffeoyl tartaric acids in cut midribs of iceberg lettuce by 92% and 98%, respectively, AIP competitively inh ibited PAL activity from a lettuce midrib homogenate with an apparent K-i of 22 nM. While the formation of phenolic compounds was strongly i nhibited by AIP, the number of lesions associated with russet spotting was not affected. Only the color of the lesions was affected by AIP. In control midribs the russet spotting lesions mere brown while those in the AIP-treated midribs were initially olive green and after 3 to 7 days these lesions turned the characteristic brown color, No tyrosine ammonia-lyase activity was detected in a homogenate of lettuce midrib tissue. These results indicate that the early development of russet s potting lesions is independent of the increase in PAL activity and phe nolic compounds rather than an effect of these increases as previously suggested, However, accumulation of phenolic compounds does contribut e to the subsequent browning symptoms indicative of russet spotting.