Developmental Control of Russet Spotting, Phenolic Enzymes, and IAA Oxidase in Cultivars of Iceberg Lettuce

Leaf midrib tissue from six cultivars of iceberg lettuce ( Lactuca sativa L.) was excised from 20- to 100-day-old plants and tested at 5C for russet spotting (RS) susceptibility under 10 μl ethylene/liter. There was no RS development in tissue excised from 20-day-old plants from any of the six cultivars. RS first developed in tissue from 50-day-old plants and tended to be more severe in tissue excised from older plants. ‘Winter Haven’ and ‘Salinas’ were most susceptible, while ‘Calmar’ was most resistant to RS; ‘Climax’, ‘El Toro’, and ‘Sea Green’ were moderately susceptible. Phenylalanine ammonia-lyase activity in the six cultivars at different developmental stages correlated with their degree of RS development. For ‘Winter Haven’, the increase in ionically bound POD and IAA oxidase activity, but not PPO activity, was associated with the increase in RS score during plant development. ‘Winter Haven’ tissue had much higher ionically bound peroxidase and IAA oxidase activities than ‘Calmar’ tissue from 100-day-old plants. Chemical names used: indole-3-acetic acid (IAA), phenylalanine ammonia-lyase (PAL), peroxidase (POD), polyphenol oxidase (PPO).