Several major changes can take place as fruits ripen. Not all occur in every type of fruit, but taken collectively they characterize ripening processes. They include: (1) a rise in respiration rate; (2) production of ethylene; (3) flesh softening; (4) appearance of colour; (5) formation of volatiles with associated development of flavour.
Among these changes, formation of bright colour, which evolved to attract dispersal agents such as birds, browsing animals and primates, now becomes a particularly important visible indicator of maturity and ripeness. Pome fruit, stone fruit, tomatoes, mangoes and straw berries provide good examples where colour is a prime indicator of ripeness.
The non-greening of leaves is called senescence. As in leaf senescence, ripening in fruits also involves chlorophyll loss and an increase in production of yellow, orange, red or purple pigments. Yellow, orange or red pigmentation, as seen in oranges and tomatoes, arise from conversion of chloroplasts to chromoplasts. In higher plants, carotenoids which are found in chromoplasts and anthocyanins which are located in the vacuoles fulfill an important purpose as colorants of fruits.
For example the red colour of tomato (Lycopersicon esculentum) fruits is due to the carotenoid pigment lycopene, whose concentration increases dramatically during the ripening process. The deep yellow colour of mango fruits is due to the presence of beta carotene a common phytochemical within a group of 600 known carotenoids. Similarly the red colour development in apples and purple colour development in grapes are due to the formation of anthocyanin pigments in their skin. Sometimes both types of pigments can occur in the same fruit.
These pigments are formed as end products of two separate pathways during ripening. The phytoene pathway, which leads to the accumulation of yellow-orange carotenoids or red lycopene and the anthocyanin pathway which leads to the accumulation of anthocyanins in the vacuoles are these two pathways.
