Ripening of Fruit

the morphological and biochemical changes in a fruit by which the seeds become viable embryos of new plants and the pericarp acquires the ability to protect and distribute the seeds.

After the flowers are pollinated, an ovary forms and begins to grow intensively. Seeds form and ripen inside the ovary, promoting the growth and maturation of the pericarp. The ripening process of the pericarp is different for dry and fleshy fruits. In dry fruits the process consists principally of the dehydration of the tissues. Thus, in Leguminosae the pericarp wrinkles and shrinks, and in Gramineae the drying pericarp concresces with the seed envelope. In fleshy fruits the pericarp proliferates at the expense of the tissues of the ovary or torus, resulting in an increase in the number and size of cells and in the formation of intercellular spaces.

There are two principal periods in the development of a fruit: the first extends from fertilization of the ovule to the maturation of the seeds and completion of the growth of the pericarp; the second lasts until complete maturation of the pericarp. The first developmental period is marked by intensified seed and pericarp growth and formation, accompanied by intensive influx of nutrient matter and water from the leaves. Synthesis of substances of high-molecular weight, such as proteins, fats, and carbohydrates (starch, cellulose, pectins), predominates in the seeds and fruit. In the second period the fruit takes on new morphological and biochemical characteristics: it becomes softer and acquires its characteristic coloring, taste, and aroma. The process of respiration, which furnishes energy to the fruit tissue, plays a large role in these changes. A characteristic feature of many types of fruits is a climacteric rise in the rate of respiration, which is observed in some fruits before picking and in others, which ripen in storage, after picking. The increase in the rate of respiration is promoted by the formation of ethylene in the fruits.

During the ripening period, the content of starch, organic acids, and phenols (tannins) decreases, whereas the amount of nitrogenous compounds and soluble sugars increases. This results in the characteristic taste of the fruit. The softening of fruits depends on changes in the ratio and properties of polysaccharides, especially pectins, in the cell walls. With the ripening of the fruit, there is a change in the composition of the pigments in the rind, flesh, and cellular juice of the fruit: chlorophyll is usually destroyed, whereas carotenoids, anthocyanins, and other pigments are synthesized. The fruit acquires its characteristic aroma by means of synthesis of alcohols, aldehydes, compound esters, and terpenes. Hormones produced by plants regulate the ripening processes. After the climacteric rise in the respiration rate, senescence and over-ripening occur.

The ripening period is long in citrus fruits and short in drupes, berries, bananas, and figs. In apples and pears this period fluctuates within a broad range, depending on the variety (summer, autumn, or winter). Two stages of ripeness are recognized in transported and stored fruits: picking and eating. The ripening of fruit is influenced by environmental factors, such as temperature, light, and composition of gases, particularly after picking.