What do they use to keep lettuce and other fresh vegetables from turning brown as fast?

Keep bottled lemon or lime juice in a spray mister in the refrigerator and spray on cut apples, avocados, peaches, pears, ecetera, to prevent these types of foods from turning brown.

Keep strawberries fresh for several days by refrigerating them, unwashed, in an airtight container between layers of paper towels.

Save those thin white plastic bags you brought ice cream home in! Once the bananas have reached the "perfect" ripeness, place them in one of those white plastic bags and store in the refrigerator. They will remain "firm textured" and retain their bright yellow color for up to a week.

To keep cut apple slices from turning brown, sprinkle them with a little lemon juice or lemon-lime soda.

Cutting salad greens with a knife may cause discoloration and bruising. Gently tearing the leaves is better and makes a more attractive salad.

To retain the white color of fresh mushrooms, slice just before using or dip in lemon juice.

http://www.cooksrecipes.com/tips/frutveg.html

Lemon juice is very helpful to the home cook dealing with fruits and vegetables, though there are some recipes to which it isn't a felicitous addition.

Here's one answer to why those pre-bagged salad greens don't turn brown quickly:
cals.arizona.edu/fps/node/26 webpage no longer exists