Handbook of Fruits and Fruit Processing

1

Physiology and Classification of Fruits

LiKuo-Tan Li

Introduction

Development of a Fruit

Pollination and Fertilization

Fruit Set

Parthenocarpy and Stenospermocarpy

Fruit Growth

Cell Division

Cell Enlargement

Seasonal Growth Curve

    Single Sigmoid Growth Pattern

    Double Sigmoid Growth Pattern

Maturation, Ripening, and Senescence

Maturation of a Fruit

Ripening of a Fruit

Senescence of a Fruit

Physiological Changes of a Fruit toward Maturity

    Color

    Seed Maturity

    Carbohydrate Profile

    Acids

    Aroma and Flavor Compounds

    Firmness

    Tannins

    Respiration

Fruit Classification

Fruits Classified by Their Origin

Fruits Classified by Respiration Rates and Ethylene Responses

Botanical Classification of Fruits

Simple Fruits

Simple Dry Fruits

    Achene

    Capsule

    Caryopsis

    Cypsela

    Fibrous Drupe

    Legume

    Nut

Simple Fleshy Fruit

    Berry

    Drupe

    Pome

    Hesperidium

    Pepo

Compound Fruits

    Aggregate Fruit

    Multiple Fruit

Accessory Fruit

Culinary Classification of Fruits

Fruits

Fruits used as Vegetables

Nuts

Cereals

References

Abstract: Fruits are an essential part of human diet and culture. Various classification systems have been applied to fruits to meet various objectives. Physiological and morphological characteristics of a given fruit species or even a given cultivar affect its postharvest life and processing quality. This chapter provides a fundamental background on how a fruit develops in the field and how fruits are categorized in modern society.

INTRODUCTION

Fruits are indispensable in human diet to supply essential vitamins, for example, vitamin A, B6, C, E, thiamine, niacin, minerals, and dietary fiber (Fourie 2001). Fruits are also savories that provide a pleasing taste. The majority of species of fruits that are grown and consumed in the modern world have been domesticated by the late Neolithic and Bronze Ages between 6000 and 3000 BC. In addition, a number of fruits that have been extensively collected from the wild by the native people were not domesticated until the early twentieth century (Janick 2005). Some other fruits, although commonly utilized by the local people, remain exotic to the rest of the world. Nowadays, fruit production and processing are among the major industries in many countries, and the trading and distribution of fruits have become an important international economic activity. Although world production and consumption of fruits have increased significantly, most people's diets still fall short of the mark set by United Nation's Food and Agriculture Organization (FAO 2003).

Botanically, a fruit is the reproductive structure of a flowering plant in which seeds form and develop. In culinary arts, fruit normally refers to an edible, juicy, and sweet entity derived from a flower on any flowering plant. Among so many species of flowering plants with so much anatomical diversity, only a relatively small group of species and fruit types are common in human diet. Nevertheless, the physiological and morphological characteristics of a given fruit species or even a given cultivar affect its postharvest life and processing quality. Therefore, it is advisable to obtain a fundamental understanding on how a fruit develops in the field and how fruits are categorized in modern society.

DEVELOPMENT OF A FRUIT

A fruit is developed from a flower and its associate tissues. The onset of fruit development begins as early as the differentiation of flower by which the apical meristem on a shoot forms a flower or inflorescence instead of a leaf or a shoot. Anatomical changes begin at the edge of the meristem, first generating the calyx and the corolla, and later the androecium (male) and the gynoecium (female) tissues. The process of flower differentiation can be completed within a few days in annual plants to nearly a year in some perennial plants. The differentiation of gynoecium continues to form the carpel or pistil in which the ovule is hosted. A gynoecium may consist of a single carpel, multiple distinct (unfused) carpels, or multiple connate (fused) carpels. Inside gynoecia ovules, within one or more ovaries develop and later become seeds upon fertilization. When mature, gynoecia may function to attract pollinators through aroma or nectar. At bloom or in some instances prior to bloom, gynoecia receive pollen grains on their specialized surface structure called a stigma and in some cases actively select genetically different pollen grains so as to avoid inbreeding. Gynoecia may facilitate the growth of pollen tube to the ovule and the delivery of sperm to the egg. The gynoecium forms the pericarp. The pericarp in most fruits differentiates into three distinct layers. The outer layer is called exocarp and normally becomes the peel of the fruit. The middle layer is the mesocarp, the major edible part of most fleshy fruits. The inner layer is the endocarp, which directly surrounds the ovary and the seed.

POLLINATION AND FERTILIZATION

Most flowering plants will not set fruit without pollination or fertilization. Pollination is the process of transferring pollen grains from anthers to stigmas. Pollination in some species occurs spontaneously at bloom due to their special structure of the flower or the specialized arrangement of their stigmas and stamens, for example, grapes and tomatoes. Pollination in most other species usually will not be completed without natural vectors, i.e., wind or insects (Stebbins 1970). The majority of common fruit crops require insect for pollination, and the pollination efficiency is usually improved by introducing bee (Apis) hives to the orchard during blooming season (Morse and Calderone 2000). Flowers of some tropical fruit crops, for example, mangos, are not attractive to bees. Instead, their major pollinators are native flies (Sung et al. 2006). In commercial production, their pollination can be benefited by introducing the oriental latrine fly (Chrysomya megacephala) to the orchard during bloom (Hu et al. 1995). Some fruits are dioecious, and pollen grains must be transferred from a male flower in a male plant to a female flower in a female plant to complete the pollination process. Examples of dioecious fruits include the wind-pollinated mulberries and the insect-pollinated kiwifruit (Hopping 1990). In monoecious fruit crops such as wind-pollinated chestnuts, walnuts, and pecans; and insect-pollinated lychee (Stern 2003), watermelons, and cucumbers, pollen grains must be transferred from a male flower to a female flower either on the same plant or on separate plants to continue the fertilization process.

Fertilization takes place after the germination of pollen grains on the stigma. A pollen grain after successful germination contains two sperm cells. Upon entering the ovule, one sperm cell fertilizes the egg cell and the other unites with the two polar nuclei of the embryo sac. The sperm and haploid egg combine to form a diploid zygote and later the embryo of the seed. The other sperm and the two haploid polar nuclei form a triploid nucleus and later the endosperm, a nutrient-rich tissue nourishing the developing embryo (Raghavan 2006). The ovary, which encompasses the ovule, develops into the pericarp of the fruit and helps to protect and disperse the seeds.

In self-fertilized fruit crops, for example, in most peach and nectarine cultivars, successful fertilization can occur within one flower, and pollen grains from other flowers or from other cultivars are not necessary (Weinbaum et al. 1986). On the other hand, fertilization in cross-pollinated fruit crops, for example, in most apple, pear, almond, and rabbiteye blueberry (Vaccinium ashei) cultivars, will not succeed with pollen grains from flowers of same cultivars or other cultivars with incompatible genetic background. Therefore, it requires the mixed planting of two genetically compatible cultivars in an orchard block to achieve a satisfactory yield (Visser and Marcucci 1984, Dedej and Delaplane 2003). In some fruit crops, for example, northern highbush blueberries (Vaccinium corymbosum), although self-pollination is possible to set fruit, cross-pollination by mixed planting two cultivars will increase the fruit size and quality (Huang et al. 1997, Ehlenfeldt 2001, Bieniasz 2007).

The embryo of the developing seed produces plant hormone gibberellins at early development stages to trigger the production of auxins and stimulates fruit growth (Ozga et al. 2002). In conclusion, pollination and fertilization are normally required to initiate fruit growth.

FRUIT SET

Fruit set refers to the retention of fruit on the plant after bloom. Most fruit crops produce numerous functional flowers but only...