oxidation.

the oxygen in air reacts with the surface of the newly cut lettuce and turns it brown.. i think the surface of the knife can have a small layer of oxidation on it too, so that can affect the lettuce as well or at least that's what i think happens....... you can get plastic lettuce knives....

Most fruits and vegetables have two chemicals called enzymes and poly-phenols. When these two chemicals combine, they "oxidizes" which turns fruits and vegetables brown. These two chemicals are usually separated by a cell wall which breaks down over time, but cutting and tearing can damage these cell walls causing browning at the edges.

Usually metal knives are thought to brown lettuce faster then tearing or using plastic lettuce knives though there is very little support for this. Either method will cause browning, and lettuce is usually consumed quickly after being cut thus browning is not an issue.