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It seems like most fruits that we consume undergo some sort of ripening process either before or after they are picked from the vine, tree, etc. I understand that sugars are released during the ripening process and this is why ripe fruits are sweeter and more palatable than non-ripe fruits.
Why, though, do fruits have to undergo this process of ripening?
Wouldn't it be simpler to just grow and already be "ripe"? Does the ripening process confer some sort of advantage to the plant?
Some of the main reasons for delayed fruit ripening are to protect immature seeds, to save energy, and incompatibility between fruit and flower morphologies.
Fruits that are sweet have generally evolved that way in order to attract animals that disperse the seeds. But, this attraction is only useful once the seeds have been fertilized and fully matured. If a plant were to develop a fruit that was immediately sweet this would attract the animals too soon and they would remove or damage the seeds before they were capable of producing a new plant.
Sweet fruits are very energetically costly. Many plants produce many more flowers than actually get successfully pollinated. It would be a great waste of energy to fill the ovaries of those flowers with sugar when the ovules might not ever get fertilized. In fact, one of the main theories for the success of flowering plants is the ability to abort fruit development in unsuccessful flowers.
Finally, in most cases having fully-formed fruits would get in the way of flower pollination. One of the most common arrangements of floral parts is to have the ovary sit between the petals and the stigma (superior ovary, hypogynous). If the ovary were very large that would render the petals useless. For plants with inferior (epigynous) ovaries, large fruits would probably ruin the arrangement of the floral display. A large fruit would also increase the distance between the stigma and the ovules, forcing pollen tubes to grow much longer, possibly reducing or delaying fertilization.
Mature, viable seeds have to form in most fruits, which involves the maturation of embryonic tissue. That can take a little time, from just a few days in some weed species to months and perhaps longer in some fruits like pineapple.