In nature, fruits develop only after the flower is effectively pollinated. If pollination does not occur, the entire flower shrivels up, and no fruit is formed. Fruit development is apparently stimulated when the developing seeds produce hormones that diffuse into the ovary wall, causing it to enlarge. Two hormones are particularly implicated in fruit formation: auxin and gibberellin. Many fruit growers routinely spray their plants with auxin to induce the formation of seedless, or parthenocarpic, fruits.
Fruit ripening is an important process that must occur properly in order for the seeds to be effectively dispersed. In fleshy fruits, such as tomatoes, cherries, apples, oranges, and bananas, fruit ripening involves several important changes in the pericarp that make the fruit more visible and palatable to a potential animal disperser. Perhaps the most visible change is in the color of the fruit. Immature fruits are green because of the presence of the pigment chlorophyll in the cells of the outer layer. Potential dispersers fail to notice the immature fruit because it blends in with the surrounding leaves. As fruits ripen, the chlorophyll breaks down, and other colors, such as orange, yellow, red, or blue, become evident. Those colors are the result of pigments that either are present in the unripe fruit and masked by the chlorophyll or develop as the fruit ripens. The texture and chemical composition of the pericarp change as well. Most fruits soften as they ripen, a result of the degradation of the cell walls in the pericarp. At the same time, starches or oils in the pericarp are chemically transformed into simple sugars such as fructose. That change causes the fruit to become better-tasting, more digestible, and thus more attractive to a hungry animal.
The physiology of fruit ripening has been well studied. Fruits such as grapes, citrus fruits, and strawberries ripen gradually. Others, such as tomatoes, apples, and pears, exhibit a transitional event called climacteric, which is marked by a dramatic increase in the rate at which oxygen is absorbed by the fruit, followed by a rapid change in the color and physical nature of the pericarp. Studies of climacteric fruit have shown that the onset of ripening can be delayed by storing the fruit at low temperatures or in an atmosphere devoid of oxygen. On the other hand, climacteric can be induced by exposing the fruit to ethylene, a plant hormone. Interestingly, ethylene is produced by ripening fruits, and thus a ripe fruit promotes the development of any unripe fruits nearby.
Finally, many plants drop their fruits at some point after they become ripened. Botanists use the term abscission to refer to the dropping process. Fruit abscission, like leaf abscission, occurs when a layer of cells at the base of the pedicel (the stem that attaches to the plant) become weakened. Studies have shown that abscission is influenced by two hormones: ethylene and auxin. Ethylene promotes abscission of fruits in many plant species, such as cherries, blueberries, and blackberries. Auxin, on the other hand, has effects that vary depending on the dose: Low concentrations promote fruit retention, while high doses cause fruits to drop.
See also: Fruit Types and Characteristics
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