Russet sprouting in lettuce is caused due to

Russet sprouting in lettuce is caused due to Ethylene exposure.
The russet spot is caused by ethylene production and can occur in mature to over-mature lettuce, especially following anaerobic conditions in the field.
Russet spotting is a physiological disorder induced in iceberg lettuce by exposure to parts per million levels of ethylene at +/-2 degrees C.
Ethylene-induced phenylalanine ammonia-lyase and ionically bound peroxidase activities correlated with the development of Russet sprouting symptoms.