As much as consumers lament the fact, tomatoes picked when green can be induced to ripen off the plant. Peppers are closely related to tomatoes, but they do not mature any further after they are removed from the plant. When picked green, tomatoes that come into contact with external ethylene (as in the food handling process), turn up their production of two enzymes involved in the production of their own ethylene which contributes to the ripening process. In peppers, externally supplied ethylene has no effect on metabolism and does not cause further maturation of the fruit. But somewhere further down in the cascade of ethylene related genes, the exposure to ethylene causes carotenoid biosynthesis and the breakdown of plant cell walls similarly as in tomatoes.
Plant Physiology, 159 (4) 1713