Learn English – What do you call a child for whom you act as a guardian / custodian

Normally, this is just referred to as browning, or that the fruit (or vegetable) has turned brown. Per the recipes area at How Stuff Works:

the browning reaction results from the oxidation of phenolic compounds in the fruit under the action of an enzyme called polyphenol oxidase (PPO), which is common in plant tissues.

Quite a late answer, but Wikipedia calls such pairs "Converses" or "Relational Antonyms"

In linguistics, converses or relational antonyms are pairs of words that refer to a relationship from opposite points of view, such as parent/child or borrow/lend. The relationship between such words is called a converse relation. Converses can be understood as a pair of words where one word implies a relationship between two objects, while the other implies the existence of the same relationship when the objects are reversed. [3] Converses are sometimes referred to as complementary antonyms because an "either/or" relationship is present between them. One exists only because the other exists.