Breeding for Fruit Quality E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1033

Veröffentlichungsjahr: 2011

Beschreibung

The global demand for high quality fruits that are rich in nutrients and that can endure the demands of worldwide supply chains is growing rapidly. Fruits are an important component of the human diet, providing vitamins, minerals, antioxidants, and fiber. All of these qualities contribute to the nutritional needs and health maintenance of humans. Breeding for Fruit Quality reviews the application of modern plant breeding methods to the development of improved varieties of fruits. Breeding for Fruit Quality opens with chapters that look at fruit biology and breeding strategies behind specific traits, including a look at traits such as organoleptic quality, nutritional value, and improved yield, among others. Subsequent chapters review breeding efforts to improve overall quality in a wide range of specific fruits. Providing broad ranging coverage of cutting edge methods now being applied to the development of fruit crops, Breeding for Fruit Quality will be a valuable resource for fruit biologists, breeders, geneticists, and industry personnel. Key Features: * Broad coverage of modern breeding technologies being implemented to improve overall fruit quality * Trait specific chapters explore efforts to promote traits of industrial and nutritional importance * Chapters in fruits, ranging from apple and grapes to tomatoes and olives, provide detailed coverage of breeding practices for economically important fruit crops

Leseprobe

Contents

Contributors

Preface

Section I Introduction

Chapter 1 The Biological Basis of Fruit QualityHarold C. Passam, Ioannis C. Karapanos, and Alexios A. Alexopoulos

Introduction

Fruit Quality

Fruit Constituents and Their Contribution to the Human Diet

Fruit Metabolism during Fruit Development, Maturation, and Ripening

Cell Wall Metabolism and Fruit Texture

The Metabolism of Volatiles that Contribute to Fruit Aroma

Pigment Metabolism and Fruit Color Changes

Respiration in Relation to Fruit Metabolism and Ripening

The Role of Ethylene in Fruit Ripening and Quality

Conclusion and Future Perspectives9

References

Section II Strategies for Improving Specific Fruit Quality Traits

Chapter 2 Fruit Organoleptic Properties and Potential for Their Genetic ImprovementDetlef Ulrich and Klaus Olbricht

Introduction

Fruit Organoleptic Properties

Organoleptic Properties during Domestication and Breeding

Flavor Diversity

Breeding for Flavor

References

Chapter 3 Breeding for Fruit Nutritional and Nutraceutical QualityJacopo Diamanti, Maurizio Battino, and Bruno Mezzetti

Introduction

The Effect of Environment and Cultivation Factors on Fruit Nutritional and Nutraceutical Quality

The Effect of Genotype on Fruit Nutritional and Nutraceutical Quality

Breeding for Fruit Nutritional and Nutraceutical Quality

Breeding Selection Strategies and Parameters for Nutritional and Nutraceutical Quality

Means to Avoiding Potential Allergens

Combining Breeding and Biotechnology for Improving Fruit Quality Fruit Nutrition and Beneficial Phytochemicals

Conclusion

References

Chapter 4 Fruit Shelf Life and Potential for Its Genetic ImprovementJosé A. Mercado, Fernando Pliego-Alfaro, and Miguel A. Quesada

Introduction

Cell Wall Composition and Structure

Cell Wall Disassembly Is the Major Determinant Factor of Fruit Shelf Life

Cell Wall Modifying Genes and Activities

Role of Turgor in Fruit Softening

Conclusion

References

Chapter 5 Breeding of Hypoallergenic FruitsZhong-shan Gao and Luud J.W.J. Gilissen

Introduction to Fruit Allergy

Fruit Allergens

Expression of Putative Allergen Genes

Selection of Hypoallergenic Variety

Genetic Modification

References

Chapter 6 Impact of Breeding and Yield on Fruit, Vegetable, and Grain Nutrient ContentDonald R. Davis

Introduction

Increasing Yield of Fruits and Vegetables

Evidence for Declining Nutrient Concentrations

The Effects of Hybridization on Yields and Nutrient Concentrations

Discussion

References

Chapter 7 Transgenic Approaches to Improve Fruit QualityYuepeng Han and Schuyler S. Korban

Introduction

Improvement of Fruit Taste

Modification of Phytonutrients Carotenoids and Flavonoids

Inhibition of Enzymatic Browning

Genetic Engineering for Seedlessness

Improvement of Firmness and Texture

Modulation of Ethylene Biosynthesis and Ripening

Modulating Interaction between Fruits and Microorganisms

Conclusion

References

Section III Improving the Quality of Specific Fruits

Chapter 8 Breeding for Fruit Quality in AppleHiroshi Iwanami

Introduction

Early Improvement and Genetic Study of the Apple

Challenge to Improve Fruit Quality

Appearance of Fruit

Eating Quality

Keeping Quality

Issues with Breeding for Fruit Quality

Conclusion

References

Chapter 9 Breeding for Fruit Quality in PrunusRodrigo Infante, Pedro Martínez-Gómez, and Stefano Predieri

Introduction

Fruit Quality

Quality Characteristics of Stone Fruits

Classical Breeding

Inheritance of Quality Fruit Traits

Molecular Breeding

References

Chapter 10 Breeding for Fruit Quality in StrawberryJeremy A. Pattison

Introduction

Sources of Variation and Genetic Improvement Strategies for Fruit Quality Traits

Conclusion

References

Chapter 11 Molecular Breeding of Grapevine for Aromatic Quality and Other Traits Relevant to ViticultureFrancesco Emanuelli, Juri Battilana, Laura Costantini, and M. Stella Grando

Introduction

The Characteristic Aroma of Muscat Varieties

Several Steps of Monoterpenoids Biosynthesis Need Further Investigations

QTL Analysis Clarifies Genetic Architecture of Mucat Flavor

The Traits of DXS

Association Mapping: A Modern Tool

Conclusion

References

Chapter 12 Breeding for Fruit Quality in MelonJuan Pablo Fernández-Trujillo, Belén Picó, Jordi Garcia-Mas, Jose María Álvarez, and Antonio J. Monforte

Introduction

Origin and Subspecific Classification

Biotechnology Tools for the Study of Fruit Quality in Melon

Fruit Quality

Perspectives

References

Chapter 13 Breeding for Fruit Quality in TomatoMathilde Causse, Rebecca Stevens, Besma Ben Amor, Mireille Faurobert, and Stéphane Muños

Introduction

Genetic Variability and Relationships between Quality Traits

QTL for Tomato Fruit Quality

MAS for Fruit Sensory Quality

Major Genes and Mutations Involved in Fruit Quality

Breeding for Nutritional Value

Conclusion

References

Chapter 14 Breeding for Fruit Quality in Pepper (Capsicum spp.)Ilan Paran and Eli Fallik

Introduction

Pepper Domestication

Fruit Morphology

Fruit Composition

Fruit Quality Disorders

Postharvest Fruit Quality

Classical Breeding for Quality

Use of Marker-Assisted Selection

Pepper Transgenics

Genetic and Genomic Resources

Future Breeding for Improved Fruit Quality

References

Chapter 15 The Time and Place for Fruit Quality in Olive BreedingLuis Rallo, Milad El Riachy, and Pilar Rallo

Introduction

The Building Blocks for Breeding: Conservation and Sustainable Use of Genetic Resources

The Concept of Quality in Olive

Breeding Olives

Conclusion

References

Chapter 16 Breeding for Fruit Quality in CitrusZiniu Deng and Juan Xu

Introduction

Fruit Coloration Improvement

Breeding for Seedless Fruits

Improving Internal Fruit Quality

Conclusion

References

Index

Color plate

Breeding for Fruit Quality

This edition first published 2011 © 2011 by John Wiley & Sons, Inc.

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Library of Congress Cataloging-in-Publication Data

Breeding for fruit quality / editors: Matthew A. Jenks, Penelope J. Bebeli.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-8138-1072-0 (hardcover : alk. paper)

1. Fruit–Breeding. 2. Fruit–Genetic engineering. 3. Fruit–Quality. I. Jenks, Matthew A. II. Bebeli, Penelope J.

SB359.35.B74 2011

634′.042–dc22

2010040941

A catalogue record for this book is available from the British Library.

This book is published in the following electronic formats: ePDF 9780470959336; Wiley Online Library 9780470959350; ePub 9780470959343

Contributors

Alexios A. AlexopoulosAgricultural University of AthensLaboratory of Vegetable ProductionAthens, GreeceJose María ÁlvarezCentro de Investigación y Tecnología Agroalimentaria de Aragón Zaragoza, SpainJuri BattilanaResearch and Innovation CenterFondazione Edmund Mach-Istituto Agrario di San Michele all’AdigeSan Michele all’Adige (TN), ItalyMaurizio BattinoDepartment of Biochemistry, Biology and GeneticsMedical School, Università Politecnica delle MarcheAncona, ItalyBesma Ben AmorINRAUnité de Génétique et Amélioration des Fruits et LégumesDomaine Saint-MauriceMontfavet Cedex, FranceMathilde CausseINRA Unité de Génétique et Amélioration des Fruits et LégumesDomaine Saint-MauriceMontfavet Cedex, FranceLaura CostantiniResearch and Innovation CenterFondazione Edmund Mach-Istituto Agrario di San Michele all’AdigeSan Michele all’Adige (TN), ItalyDonald R. DavisBiochemical Institute (retired)The University of Texas at AustinAustin, Texas, USAZiniu DengNational Center for Citrus Improvement, ChangshaHunan Agricultural UniversityHunan, ChinaJacopo DiamantiDepartment of Environmental and Crop ScienceUniversità Politecnica delle MarcheAncona, ItalyMilad El RiachyDepartamento de AgronomíaUniversidad de CórdobaCampus de RabanalesEdificio Celestino MutisCórdoba, SpainFrancesco EmanuelliResearch and Innovation CenterFondazione Edmund Mach-Istituto Agrario di San Michele all’AdigeSan Michele all’Adige (TN), ItalyEli FallikAgricultural Research OrganizationThe Volcani CenterBet Dagan, IsraelMireille FaurobertINRAUnité de Génétique et Amélioration des Fruits et LégumesDomaine Saint-MauriceMontfavet Cedex, FranceJuan Pablo Fernández-TrujilloDepartment of Agricultural and Food EngineeringTechnical University of CartagenaCartagena (Murcia), SpainZhong-shan GaoDepartment of Horticulture Allergy Research CenterZhejiang UniversityHangzhou, ChinaJordi Garcia-MasCentre de Recerca en Agrigenòmica CSIC-IRTA-UABCabrils (Barcelona), SpainLuud J.W.J. GilissenPlant Research InternationalWageningen University and Research CentreWageningen, The NetherlandsM. Stella GrandoResearch and Innovation CenterFondazione Edmund Mach-Istituto Agrario di San Michele all’AdigeSan Michele all’Adige (TN), ItalyYuepeng HanKey Laboratory of Plant Germplasm Enhancement and Specialty AgricultureWuhan Botanical GardenChinese Academy of SciencesMoshan, Wuhan, ChinaRodrigo InfanteDepartamento de Producción AgrícolaUniversidad de Chile, Santiago de ChileHiroshi IwanamiApple Breeding and Physiology Research TeamNational Institute of Fruit Tree ScienceMorioka, JapanIoannis C. KarapanosAgricultural University of AthensLaboratory of Vegetable ProductionAthens, GreeceSchuyler S. KorbanDepartment of Natural Resources and Environmental SciencesUniversity of IllinoisUrbana, Illinois, USAPedro Martínez-GómezDepartment of Plant BreedingCEBAS-CSICMurcia, SpainJosé A. MercadoInstituto de Hortofruticultura Subtropical y Mediterránea “La Mayora”(IHSM–UMA–CSIC)Departamento de Biología VegetalUniversidad de Málaga, SpainBruno MezzettiDepartment of Environmental and Crop ScienceUniversità Politecnica delle MarcheAncona, ItalyAntonio J. MonforteInstituto de Biología Molecular y Celular de PlantasUniversidad Politécnica de Valencia-Consejo Superior de Investigaciones CientíficasValencia, SpainStéphane MuñosINRAUnité de Génétique et Amélioration des Fruits et LégumesDomaine Saint-MauriceMontfavet Cedex, FranceKlaus OlbrichtHansabred GmbH & Co. KGThe Strawberry Breeding CompanyDresden, GermanyIlan ParanAgricultural Research OrganizationThe Volcani CenterBet Dagan, IsraelHarold C. PassamAgricultural University of AthensLaboratory of Vegetable ProductionAthens, GreeceJeremy A. PattisonDepartment of Horticultural ScienceNorth Carolina State UniversityPlants for Human Health InstituteKannapolis, North CarolinaBelén PicóInstitute for the Conservation and Breeding of Agricultural BiodiversityUniversidad Politécnica de Valencia, SpainFernando Pliego-AlfaroInstituto de Hortofruticultura Subtropical y Mediterránea “La Mayora”(IHSM–UMA–CSIC)Departamento de Biología VegetalUniversidad de Málaga, SpainStefano PredieriIBIMET-CNRIstituto di BiometeorologiaDipartimento AgroalimentareBologna, ItalyMiguel A. QuesadaInstituto de Hortofruticultura Subtropical y Mediterránea “La Mayora”(IHSM–UMA–CSIC)Departamento de Biología VegetalUniversidad de Málaga, SpainLuis RalloDepartamento de AgronomíaUniversidad de CórdobaCampus de RabanalesEdificio Celestino MutisCórdoba, SpainPilar RalloDepartmento Ciencias AgroforestalesETSIA-Universidad de Sevilla, SpainRebecca StevensINRAUnité de Génétique et Amélioration des Fruits et LégumesDomaine Saint-MauriceMontfavet Cedex, FranceDetlef UlrichJulius Kühn-InstituteInstitute for Ecological Chemistry, Plant Analysis and Stored Product ProtectionQuedlinburg, GermanyJuan XuNational Key Laboratory of Crop Genetic Improvement Huazhong Agricultural UniversityWuhan, China

Preface

The importance of fruit quality in fresh-picked produce and the ability to maintain high quality during storage is becoming increasingly important as producers seek to reduce the cost of energy inputs while working to feed a growing world population. Fruits are defined botanically in this book as the mature or ripened ovary wall of the flower and include a collection of many important food crops, such as apples, cherries, peaches, plums, apricots, grapes, blueberries, currants, cranberries, strawberries, raspberries, olives, avocados, bananas, pineapples, oranges, lemons, limes, grapefruits, kiwifruits, pomegranates, mangos, guavas, figs, tomatoes, peppers, eggplants, cucumbers, pumpkins, watermelons, cantaloupes, squashes, and many others. Although nuts, grains, and some spices are derived from botanic fruits, only the fleshy edible fruits are considered in these chapters. The size of the fruit industry worldwide, as defined, is difficult to estimate but is well into the hundreds of billions, with tomato leading in tonnage produced (i.e., 125 million metric tons per year), followed in order by citrus, watermelon, banana, grape, and apple (FAO Statistics, 2005). Cucumbers, eggplants, cantaloupes and melons, mangos, and peppers also have prominent world market share.

Fruits are important to the human diet, contributing among other health benefits significant amounts of vitamins, minerals, antioxidants, and fiber. Fruits are important to human health because people groups from around the world that have limited access or otherwise consume less fruits are predisposed to numerous health problems including increased rates of cancer, cardiovascular diseases, and neurological and macular degeneration. Fruits, however. are high priced commodities in most parts of the world, and because of this, are scarce in diets of many having low incomes. Increasing the access to high quality fruit is thus of critical importance to sustain the health and well-being of our world’s growing populations.

Recent reports suggest that contemporary production practices and crop breeding strategies to increase fruit yield have contributed little to increasing the actual nutritional value of fruits. For example, the nutrient content of many commodities has declined due to modern production systems managed to grow large fruit rapidly, in what has been called a nutrient dilution effect. In addition, many new cultivars selected for high yield, apart from selection for overall nutrient content, also exhibit significant nutrient decline. New production practices that promote earlier harvesting (before full ripening) and modern commodity distribution systems that require long distance shipping and long-term storage of fruits can also cause a decline in fruit nutrition and other important fruit quality attributes.

A renewed effort worldwide to improve fruit yield and quality is underway, as evidenced in the new specialty crop research initiatives being supported by many national and international agricultural agencies. Although great advances are expected from research to improve fruit quality through modification of both field-production and postharvest practices, the potential to apply modern breeding approaches to improve fruit characteristics such as appearance, organoleptic properties, nutritional value, trait homogeneity, and storage life is an exciting area of research whose possible impacts on world food systems have only begun to be explored.

This book will present a thorough analysis of the plant breeding efforts ongoing worldwide to improve fruit quality, as well as a contemporary understanding of the physiological, biochemical, developmental, and genetic mechanisms underlying the associated traits. Chapters in this book will examine new strategies being employed to identify and then enhance fruit characteristics, including efforts to discover existing genetic variation in crop germplasm and wild relatives, and to manipulate genetic variation using classical, transgenic, and molecular marker-assisted breeding approaches. This book seeks to integrate discussion of these modern crop improvement strategies and expertise with recent advances in our understanding of the key biological determinants of fruit quality. Information presented here will be especially useful to agronomists and horticulturists, crop breeders, molecular-geneticists, and biotechnologists and serve as an important scholarly text for educators, postgraduate students, and researchers.

We, the editors, would like to give a special thanks to the authors for their outstanding and timely work in producing such excellent chapters. We would also like to thank Wiley-Blackwell Publishing’s Justin Jeffryes for his advice and encouragement during the development process. And finally, we thank the Fulbright Foundation – Greece for bringing the two editors together in Athens to plan and produce this important book, “Breeding for Fruit Quality”.

Matthew A. Jenks and Penelope J. Bebeli

Breeding for Fruit Quality

Section IIntroduction

1 The Biological Basis of Fruit Quality

Harold C. Passam, Ioannis C. Karapanos, and Alexios A. Alexopoulos

Introduction

The fruits of cultivated plants may be grouped into two broad categories: dry fruits (nuts and grains), which are grown almost invariably for their seed, and fleshy fruits in which the succulent pericarp normally comprises the major nutritive tissue. Although the word fruit popularly refers to fleshy fruit, the vast majority of fruits in nature are dry when mature, as are cultivated staple grains, such as wheat (Triticum spp. L.) and barley (Hordeum vulgare L.), where botanically the “seed” is a caryopsis with the testa (seed coat) fused to the pericarp. Here, however, we are concerned only with fleshy fruit, and for practical purposes we group them into three categories:

(1) Dessert fruit, for example, oranges (Citrus × sinensis [L.] Osb.), apples (Malus domestica Borkh.), and grapes (Vitis vinifera L.).

(2) Salad fruit, for example, tomatoes (Solanum lycopersicum L.) and cucumbers (Cucumis sativus L.).

(3) Vegetable fruit, for example, fruit that are normally cooked before consumption, such as aubergines (Solanum melongena L.) and marrows (Cucurbita spp. L.).

This classification is not exclusive, and some fruit species may belong to more than one category. For example, because of their soft structure and high perishability, berries (cranberries [Vaccinium spp. L.] and blackberries [Rubus spp. L.]) are frequently classified as “soft fruit”, whereas tomato is consumed both as a salad fruit and after cooking.

Fruit Quality

Quality is a term that when applied to fresh fruit may convey a number of interpretations. Particularly within the markets of Europe, North America, and Australia, quality refers to the external appearance of the product. Within the European Union (EU), quality standards are applied to fresh fruit and vegetables and adhere to obligatory standards within the member states (EU, 2008). Additionally, produce exported to the EU from countries outside Europe must conform to the EU standards. In general this means that according to their grade (i.e., extra, first, and second) fruit and vegetables are packed and graded so as to be virtually free of injury, blemishes, and disease and to be uniform in shape size, color, and maturity. Similar standards in the United States are set by the United States Department of Agriculture (USDA).

Quality, however, does not only relate to appearance, and even the most rigid application of the EU quality standards does not ensure that a particular fruit or vegetable will be tasty, rich in nutrients and vitamins, or that it will ripen to be sweet and aromatic. Moreover, the EU standards do not take into account local preferences within regional markets. Indeed, local preferences may vary widely between member states and within these states. For example, some markets prefer large (beefsteak) tomatoes; others, small tomatoes. In some markets the tomato calyx must remain attached to the fruit throughout handling and marketing; in others, the calyx is removed at harvest. The degree of acceptability of mechanical damage or shape irregularity also varies between markets.

Quality should be a prime target for plant breeders, but how universally applicable are their objectives and what markets do they aim at? Until the 1960s, fruit and vegetable seed production was in the hands of relatively small seed houses aiming largely at local markets. This meant that there was a rich biodiversity of fruit and vegetable crops and growers could select varieties or hybrids that were both suitable to their local growing conditions and that produced products that were desired within the local markets. However, since the intervention of large multinationals in the breeding and seed production industry virtually all this has changed. Economic gain forestalls traditional cultivation methods and establishes new patterns of market acceptance. Biodiversity is not seen by multinational directors as conducive to shareholders’ profits. Instead, the fewer the number of varieties of tomato and the wider their distribution worldwide, the higher the profits are likely to be. Modern tomato cultivars may be grown equally well in virtually any region of the world given the know-how. They may comply 100% with the EU or USDA quality standards, be of beautiful appearance, and as uniform as “peas in a pod”, but with indifferent texture, aroma, and flavor and low nutritional value. Therefore, in considering the biological basis of quality, we shall concentrate primarily on organoleptic and nutritional quality traits, particularly the constituents of fruits that are important for a healthy, human diet, and the biological processes involved in their metabolism. The way in which breeding contributes to quality forms the subject of the subsequent chapters.

Fruit Constituents and Their Contribution to the Human Diet

Some fruits (e.g., tomatoes, bananas [Musa spp. L.], oranges, and apples) are consumed widely throughout the whole world, whereas others are more localized in demand (e.g., olives (Olea europaea L.), berries). Moreover, the increase in global travel and communication has raised consumer awareness of fruits, which until just a few decades ago were virtually unknown. However, although the consumption of fresh fruits within the western world has tended to increase over recent decades, per capita consumption is frequently lower than that recommended for a healthy diet and varies with consumer habits and the availability of supply (Lock et al., 2004).

Fruits are an important natural source of essential vitamins, in addition they contain water, organic acids, fats, carbohydrates, proteins, fiber, antioxidants, and inorganic minerals (Lock et al., 2004). The concentrations of these substances vary among species and cultivars and can be influenced by environmental factors, cultivation practices, and postharvest handling (Kays, 1991). Recently, emphasis has been placed on the occurrence of antioxidants because these play a crucial role in removing reactive oxygen species (free radicals), such as singlet oxygen (O−), hydrogen peroxide (H2O2), superoxide (O2−), or hydroxyls (OH−) (Asada, 1999), which may cause oxidative damage to cells and are implicated in chronic illnesses, such as cancer and cardiac disease (Mittler, 2002). Apart from vitamins A and C, a number of other fruit constituents (e.g., flavonoids and phenolics) have significant antioxidant properties and are considered to be of particular value to human health.

Vitamins

Vitamins are organic molecules that are essential in trace amounts for human metabolism. They may be grouped into six categories: A, B complex, C, D, E, and K. Fresh fruits contain significant amounts of vitamins A, B, C, and E.

Vitamin A is essential for the functioning of the retina of the eye and is crucial for normal vision. Deficiency leads to impaired vision and even blindness (Rice et al., 2004). In developing regions of the world, vitamin A deficiency is estimated to cause blindness in 250,000 to 500,000 children each year. Additionally, it plays an important role in gene transcription, cell division and differentiation, reproduction, and the maintenance of normal skin health, as well as being a powerful antioxidant (Rice et al., 2004). Vitamin A can be of animal or plant origin. In plants, the carotenoids (i.e., α-carotene, β-carotene, γ-carotene, and the xanthophyll, β-cryptoxanthin) function as precursors of vitamin A. The human organism requires about 700 (female adults) to 900 µg (male adults) vitamin A per day. Fruits that are particularly good sources of provitamin A include cantaloupe melon (Cucumis melo var. cantalupensis Naud.), apricot (Prunus armeniaca L.), papaya (Carica papaya L.), and mango (Mangifera indica L., 40–170 µg 100 g−1 fresh weight) (Kays, 1991).

The B vitamins (i.e., B1, B2, B3, B5, B6, B7, B9, and B12) are a group of water-soluble compounds that contribute to human health by supporting cell growth and metabolism, skin and muscle tone, the function of the immune system, erythrocyte metabolism, and the prevention of anemia. With the exception of vitamin B12, all the other B complex vitamins are available from plant sources; for example, avocado (Persea americana Mill.) contains vitamins B2, B3, B5, B6, and B9; chili pepper (Capsicum spp. L.) contains vitamins B2 and B6; okra (Abelmoschus esculentus [L.] Moench.) contains vitamins B1 and B9; and banana contains vitamins B3, B5, and B6. A regular intake of the B vitamins is required because any excess is excreted in the urine. A lack of B vitamins is associated with various skin disorders and dermatitis, as well as diseases such as beriberi (B1 [thiamine]), hyperemia (B2 [riboflavin]), and anemia (B6 [pyridoxine], B9 [folic acid], and B12 [cobalamin]).

Vitamin C (L-ascorbic acid) is a water-soluble sugar-lactone and a strong antioxidant. In humans it acts as an enzyme cofactor for biosynthetic reactions, a substrate for ascorbate peroxidase, and an electron donor for certain enzymes (Hancock & Viola, 2005). Vitamin C (in the form of lime juice) was used to prevent scurvy among seamen long before its isolation in 1932. Moreover, patients suffering from oxidative stress, such as that related to cardiovascular disease, hypertension, chronic inflammatory disease, and diabetes, exhibit a lower plasma ascorbate concentration (45 µmol l−1) than that of healthy individuals (61.4–80 µmol l−1) (Schorah et al., 1996). Fruits that are particularly rich in vitamin C include blackcurrant (Ribes nigrum L., 155–215 mg 100 g−1 fresh weight), pepper (Capsicum annuum L., 134–155 mg 100 g−1 fresh weight), kiwi (Actinidia deliciosa A. Chev., 65–100 mg 100 g−1 fresh weight), and citrus (Citrus spp. L., 65–85 mg 100 g−1 fresh weight) (Kays, 1991).

Vitamin E is a generic name for tocopherols (i.e., α-, β-, γ-, and δ-tocopherols) and tocotrienols, which are lipophilic antioxidants considered to be important for the removal of reactive oxygen species created during lipid oxidation and for the protection of cell membranes and the reduction of blood cholesterol levels. The recommended daily intake of vitamin E (α-tocopherol) by adult males or females is 15 mg (22.4 IU). Although the most valuable natural sources of vitamin E for the human diet are wheat germ, nuts, and vegetable oils (Kays, 1991), significant amounts can be derived from a number of fresh fruits, including olives and avocados.

Vitamin K (K for Danish, koagulation) denotes a group of lipophilic, hydrophobic nutritional factors required for blood clotting and other metabolic processes relating to vascular biology and bone metabolism of which vitamin K1 (also named phylloquinone, phytomenadione, or phytonadione) is found in a number of green plants and fruits. The recommended daily intake of vitamin K is 90 µg for adult females and 120 µg for adult males. As in the case of other lipophilic vitamins (i.e., A, D, and E), vitamin K is stored in the fat tissue of the body. Fruits with high vitamin K levels are kiwi (34–50 mg 100 g−1 fresh weight), blueberry, blackberry (Vaccinium spp. L., 15–27 mg 100 g−1 fresh weight) and grape (Vitis vinifera L., 14–18 mg 100 g−1 fresh weight) (MacKenzie et al., 2003).

Fiber

Fibers formed from macromolecules, such as cellulose, hemicelluloses, pectins, lignin, resistant starch, and nondigestible oligosaccharides, are important for the proper function of the peptic system. Fibers are not digested within the human gut, but by adding bulk, they shorten the transit time through the intestinal tract and regulate bowel function (Anderson & Chen, 1979). Soluble fibers may absorb water to become gelatinous and fermentable by bacteria. They can also bind bile acids, thus restricting their entry into the body and reducing cholesterol levels, as well as regulating blood sugar levels and balancing intestinal pH. Lignin is also believed to have antioxidant properties. The human fiber requirement is estimated to be about 20 to 35 g per day. Although this may be provided mainly by the ingestion of vegetables, fruits also contribute about 1 to 3% of their fresh mass, with 30 to 35% in the form of cellulose, 25 to 35% hemicelluloses, and 20 to 35% pectins (Marlett, 1992).

Phenolic Compounds

Phenolics are substances (nearly 10,000) with aromatic rings and variable degrees of hydroxylation (Taiz & Zeiger, 2002; Mattila et al., 2006). Some phenolic compounds are carboxylic acids soluble in water; some are soluble only in organic solvents, whereas others are insoluble polymers (Taiz & Zeiger, 2002). Phenolic compounds are invariably present in small amounts in fruits, but in strawberries can constitute as much as 0.1% of their fresh mass. In general, phenolic compounds are located more in the peel than in the pulp. They may be grouped into two main categories: phenolic acids and flavonoids, as well as other substances such as lignans, stilbenes, tannins, and coumarines.

Phenolic acids include the products of benzoic and cinnamic acid, such as p-hydroxybenzoic, vanillic, syringic, gallic acid, and p-coumaric, caffeic, ferulic, and sinapic acid, respectively. The antioxidant properties of phenolic acids vary and depend on the structure of their molecule. Some fruits, especially berries, are rich in caffeic acid (Mattila et al., 2006).

Flavonoids are low-molecular-weight polyphenolic compounds that may be grouped as flavones and flavonols, flavanones and flavanols, isoflavones, proanthcyanidins, and anthocyanidins (Le Marchand, 2002). Flavones (e.g., rutin, luteolin, and apigenin) and flavonols (e.g., quercetin and kaempferol) are present in high amounts in blueberries and citrus, especially in the peel. Flavanones are present in citrus (e.g., hesperidine, which is a glycoside form) (Tripoli et al., 2007), whereas flavanols, such as catechin and epicatechin, are present in grapes (Rice-Evans et al., 1997). Isoflavones, such as genistein, glycitein, and daidzein, are present mainly in legumes, whereas proanthocyanidins (derived from catechin and epicatechin) are present in grapes, apples, and blueberries (Gu et al., 2004). Anthocyanidins (e.g., pelargonidin, cyanidin, delphinidin, peonidin, petunidin, and malvidin) are always present in the glycoside form, which has fewer antioxidant properties than the nonglycoside form. The concentration of flavonoids is invariably higher in mature fruits, and they may be present only in the peel or in the flesh of the fruit as well, depending on the plant species and variety. For example, red-flesh varieties have a higher flavonoid content than white-flesh ones.

Lignans are usually present in small amounts in fruits, with the exception of legume fruits. Resveratrol is a stilbene that is present in grapes, especially when they are produced under stress conditions (Langcake & Pryce, 1976) and in blueberries.

Organic Acids

Organic acids in fruits may be: (a) monocarboxylic acids (i.e., formic, acetic, and butyric acids), monocarboxylic acids with alcohols (i.e., glycolic, lactic, glyceric, and mevalonic acids), ketones (i.e., pyruvic acid), or aldehydes (i.e., glycoxylic acid); (b) di- or tricarboxylic acids (i.e., oxalic, succinic, fumaric, malic, tartaric, citric, and isocitric acids); (c) acids derived from sugars (i.e., saccharic, galacturonic, and glucuronic acids); or (d) cyclic monocarboxylic acids, such as aromatic acids (i.e., benzoic, salicylic, and caffeic acids) or alycyclic acids (i.e., quinic and shikimic acids). They possess acidic properties due to the presence of their carboxyl (COOH) group(s), exist either as free acids or anions or in the form of esters and glycosides, and are located in active pools within the cytoplasm that contribute to cellular metabolism or are stored within the cell vacuole. Additionally, some acids may exist in the form of insoluble salts (e.g., oxalates) (Kays, 1991).

Apart from their role in cell metabolism (i.e., as components of the tricarboxylic acid cycle or in photosynthesis), organic acids significantly contribute to the flavor and aroma of fresh fruit. Although most organic acids within fruits are present only in trace amounts, some occur in much larger concentrations. For example, citrus fruit (e.g., oranges, lemons [Citrus × limon L.]) contain particularly large amounts of citric acid; apples, pears (Pyrus communis L.), and peaches (Prunus persica [L.] Batsch.) contain mainly citric and malic acids, whereas in grapes tartaric and malic acids predominate. In other fruits (e.g., bananas, cranberries), quinic acid and benzoic acid, respectively, are important aromatic constituents (Kays, 1991; Vicente et al., 2009).

Proteins

Although the protein and amino acid content of fresh fruit is rather low (typically <1% fresh weight), these components play a significant role in fruit maturation and ripening. In leguminous species (e.g., beans [Phaseolus vulgaris L.]) the protein content of the pods may be as high as 30%, due mainly to protein accumulation in the seed (Vicente et al., 2009), whereas in climacteric fruit, such as apple and avocado, the protein concentration increases during the early stages of ripening. The newly synthesized proteins play a direct role in the ripening process because inhibition of protein synthesis also inhibits ripening.

Lipids and Fatty Acids

Lipids are composed of long-chain fatty acids that may be saturated (i.e., lauric, myristic, palmitic, stearic, behenic, and lignoceric acids), monounsaturated (i.e., palmitoleic, cis-vaccenic, and oleic acids), or polyunsaturated molecules (i.e., arachidonic, linoleic, or α-linolenic acids) (Kays, 1991).

Lipids function as storage reserves (e.g., fatty acids and triacylglycerols or triglycerides) or as structural components of biological membranes (e.g., glycerol-phospholipids, glyceroglycolipids, sphingolipids, and sterols) (Taiz & Zeiger, 2002). Esterified fatty acids coupled with one of the three hydroxyl groups of the glycerol molecule can form natural triglycerides (Mazliak, 1970), resulting in oil inclusions in the cell. Additionally, the cuticle, which protects the outer tissue layers of fruit and other plant organs, contains waxes (typically high molecular weight esters of fatty acids and higher alcohols) the composition of which is important for the restriction of water loss and protection against mechanical damage and pathogens (Mazliak, 1970).

Lipids in fresh fruits usually account for less than 1% fresh weight, with the exception of avocado (4–20% fresh weight) and olive (15–40% fresh pulp weight), but with significant variation between cultivars (Mazliak, 1970). The monounsaturated fatty acids are considered to be of great importance for decreasing the level of low-density lipoprotein (LDL) cholesterol (the so called “bad” cholesterol) in the blood. Generally, fatty acids affect arterial blood pressure and the response of the organism to inflammation. The occurrence of linoleic acid and α-linolenic acid is important because these acids cannot be synthesized in the human body. Moreover, these polyunsaturated fatty acids are enlisted as omega fatty acids, which are considered to be important for human health.

Carbohy drates

Carbohydrates are the principal storage components of most fruit and may constitute between 3 and 20% fresh weight (e.g., cucumber 3.5%, watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] 10–14%). In addition to acting as important energy reserves and substrates for respiration, carbohydrates constitute important structural components of the cells. During fruit maturation and ripening, significant changes in the carbohydrate composition (e.g., starch breakdown into sugars, hydrolysis of pectins) lead to fruit softening and sweetening, which are essential for fruit quality. Carbohydrates are present in the form of polysaccharides, oligosaccharides, and monosaccharides. Glucose, fructose, and sucrose are the most abundant water-soluble carbohydrates in fruits and are the principal sources of sweetness (Taiz & Zeiger, 2002). However, the relative concentrations of individual sugars vary greatly between species and cultivars, as well as with the stage of maturation and ripening. For example, in apple, pear, strawberry (Fragaria × ananassa Duch.), and grape, the concentrations of glucose and fructose are higher than that of sucrose, whereas in banana, pineapple (Ananas comosus [L.] Merr.), peach, and melon (Cucumis melo L.), the major soluble sugar at ripeness is sucrose. During fruit growth and maturation, starch may accumulate (e.g., in tomato), but at full ripeness the starch content is relatively low.

Minerals

Fresh fruits are a good source of minerals, many of which are considered to play an important role in human health.

Nitrogen, which is present in all fresh fruits, is essential for protein and amino acid synthesis. Nitrogen intake by humans may be from water, animals, or plants. However, the occurrence of high nitrate concentrations within some plant products (particularly leafy vegetables, but also some fruit) has been implicated in the occurrence of methemoglobinaemia in infants and the formation of potentially dangerous carcinogens within the digestive tract (Sanchez-Echaniz et al., 2001; Weyer et al., 2001). On the other hand, there may also be a beneficial role for nitrates and nitrites in human health, where according to Dietary Approach to Stop Hypertension (DASH), diets rich in vegetables and even exceeding the recommended daily intake of nitrates cause vasodilation, decrease blood pressure, and support cardiovascular function (Hord et al., 2009).

Phosphorus participates in vital cellular functions (e.g., DNA and RNA synthesis) and is considered to be an essential mineral for human growth. Although human phosphorus intake occurs mainly from nonplant sources, fresh fruits may also contribute (e.g., avocado, kiwi, raspberry [Rubus idaeus L.], strawberry, and apricot).

Potassium is an important electrolyte concerned with the regulation of blood pressure and heartbeat (McCarron & Reuser, 2001), the maintenance of bone mass during aging (McDonald, 2007), and the release of energy from carbohydrates, proteins, or fats (Ignarro et al., 2007). Although it is present in all plant organs, fresh fruits such as avocado, apricot, banana, kiwi, fig (Ficus carica L.), pomegranate (Punica granatum L.), orange, and melon are considered to be among the richest sources.

Calcium is essential for bone and tooth formation, and calcium deficiency increases the risk of osteoporosis (McCarron & Reuser, 2001). Human calcium intake occurs largely from dairy products, but it is present in relatively high amounts in fresh fruits such as orange, fig, kiwi, lemon, and blackberry.

Magnesium participates in protein synthesis, enzyme activation, body temperature regulation, and bone formation (together with other minerals). Raspberry, avocado, banana, and blackberry are good sources of magnesium intake.

Plant micronutrients (i.e., iron, copper, manganese, zinc, sulfur, selenium, cobalt, sodium, chlorine iodine, and fluorine), although present only in trace amounts, are also important for the human diet.

Iron is an essential component of hemoglobin and also takes part in human body functions, such as the immune response, bone structure, and protein and enzyme structure (Arredondo & Núñez, 2005). Avocado, berries, lemon, cherry (Prunus cerasus L.), fig, and grape are good sources of iron.

Copper is important for some protein functions and for the formation of hemoglobin. It affects the progression of cardiovascular disease and diabetes, and in the case of deficiency during pregnancy, it can lead to structural malformations in the fetus and persistent neurological and immunological abnormalities in the offspring (Uriu-Adams & Keen, 2005). Avocado, blackberry, kiwi, grape, and fig are valuable sources of copper for the consumer.

Manganese participates in bone structure, brain function, blood sugar regulation, and is a cofactor for certain antioxidant enzymes. It occurs in relatively high amounts in avocado, banana, various berries, and pineapple.

Zinc is considered to have antioxidant properties and may affect the immune response. It also affects the structure and activity of certain enzymes. Avocado, blackberry, raspberry, and fig are relatively good sources of zinc.

Sulfur and sodium, both essential for human metabolism, are supplied primarily by water and the environment, although melon (cantaloupe) and avocado have relatively high concentrations of sodium.

Fruit Metabolism during Fruit Development, Maturation, and Ripening

Ripening is an essential process for the development of fruit quality, but in view of the wide range of fruit types, it is hardly surprising that differences are observed with respect to ripening metabolism even though the central biochemical pathways are common.

Fruit development and growth are dependent on photosynthetic CO2 fixation in leaves and the translocation of sugars (i.e., sucrose, in particular, stachyose, raffinose, and sorbitol), amino acids, and organic acids to the fruit cells (Ho, 1988). During the early phase of development, most fruits, like meristems, can be regarded as utilization sinks because of their high metabolic activity and rapid cell division. During the later phase of development, which is characterized by cell expansion, seed development and maturation, most fruits accumulate high levels of carbohydrates in the form of starch or sugars and are thus more typical of storage sinks (Ho, 1988).

Our perception of flavor relies on two senses, taste and smell, and taste sensations may be characterized as sweet, sour, or bitter (salty not contributing to the flavor of fresh fruit). Fruit flavor, however, is also affected by our sense of smell and the presence of specific flavor volatiles. Hence, the flavor and aroma profile of an individual fruit derives from a complex interaction of sugars, organic acids, phenolics, and more specialized flavor compounds, including a wide range of volatiles (Tucker, 1993).

Carbohydrate Metabolism

Although most fruit cells have functional chloroplasts during their development, they do not appear to contribute significant amounts of photo-assimilates to fruit growth (Gillaspy et al., 1993), and so fruit sugars and organic acids mainly originate from leaf photosynthetic assimilates. Most fruit accumulate the bulk of their carbohydrate before the onset of ripening. Growing tomato and banana fruits tend to accumulate high amounts of starch (up to 20% of dry weight in young tomato fruit), which is degraded at the later stages of ripening to produce sucrose and hexoses (Seymour, 1993; Kanayama & Odanaka, 2000). Citrus fruits following the same pattern accumulate glucose, fructose, and sucrose as they ripen (Ting & Attaway, 1970). These fruits can be harvested at the mature green stage and still attain acceptable flavor on ripening after storage (Tucker, 1993). Other fruits, however, continue to accumulate sugar from the plant during ripening (e.g., strawberry, grape). Sucrose translocated into grapes is hydrolyzed, so that the ripe fruits contain mainly glucose and fructose and only small amounts of other sugars (e.g., less than 0.1% sucrose) (Kanellis & Roubelakis-Angelakis, 1993). Cucurbits also do not accumulate starch and require active translocation of photo-assimilates from leaves during development and ripening (Handley et al., 1983). For satisfactory flavor, cantaloupe melons should contain at least 10% sugar, and if harvested before maturity, the fruits of these species (also grape, strawberry, watermelon and muskmelon [Cucumis melo L.]) are insipid and not sweet even when ripe (Bianco & Pratt, 1977). Even in fruits that accumulate starch, such as tomato, the longer the fruit remain on the plant before harvest the better the flavor will be (Tucker, 1993), as indicated by differences in acid-to-sugar ratios and volatile profiles between fruit ripened on the plant and those ripened after harvest at the green stage.

Tomato is a good example of a fruit that stores carbohydrate in the form of starch before ripening. Sucrose is translocated to the fruit and metabolized by acid invertase and possibly sucrose synthase (SuSy) so as to maintain a low fruit sucrose concentration and therefore a steep sucrose concentration gradient between the phloem and the surrounding cells within the fruit. Glucose and fructose derived from sucrose are phosphorylated by hexokinase and fructokinase respectively for further metabolism, such as glycolysis and starch synthesis. Two divergent fructokinase genes, Frk1 and Frk2, have been shown to be expressed differentially in tomato plants, with Frk1 playing a role as a “housekeeping enzyme” in carbohydrate metabolism in all plant cells, whereas Frk2 is induced for starch synthesis and seed development (Kanayama & Odanaka, 2000).

By contrast, in young cucumber and melon fruits α-galactosidase cleaves galactose from imported stachyose and raffinose to leave sucrose; whereas galactose is further metabolized via galactokinase, UDP-Gal pyrophosphorylase (PPase), and UDP-Glc-4 epimerase to form UDP-glucose (Gao & Schaffer, 1999). In pome fruits, the translocated assimilates (70% sorbitol and 30% sucrose) are metabolized to form fructose, glucose, sucrose, malic acid, and starch. Sorbitol in apples and Japanese pears (Pyrus pyrifolia [Burm.] Nak.) is converted to fructose by sorbitol dehydrogenase; thus in these fruits, fructose is produced in preference to glucose (Berüter, 2004). Sorbitol oxidase has also been detected as a minor cell-wall bound enzyme in apples (Knee, 1993).

During the later stages of ripening of fruits that accumulate starch, rapid hydrolysis of starch is catalyzed by the enzymes α- and β-amylase and starch phosphorylase. In banana, starch reserves decrease from 25% to less than 1% of the total fruit weight during the climacteric phase, whereas the sucrose content increases 12-fold followed by an increase in hexoses (Cordenunsi & Lajolo, 1995). The enzymes responsible for starch degradation are active only against the linear glucose chains of amylose within the starch and are unable to degrade the α(l–6) branch points also found in the amylopectin of starch. However, enzymes capable of attacking the branch points (i.e., debranching enzyme, EC 3.2.1.10) have been identified in several tissues, including banana. Although starch phosphorylase hydrolyzes the terminal α(1–4) linkage to give glucose-1-phosphate, amylases produce maltose, which is coverted into glucose by the action of α-1,4- and α-1,6-glucosidases. The end products of starch degradation (i.e., glucose and glucose-1-phosphate), are converted to glucose-6-phosphate, by the action of hexokinase or glucose phosphate mutase respectively. Starch degradation is confined to the plastids, but utilization of the breakdown products occurs mainly in the cytoplasm, either in respiration or by reconversion to glucose phosphate and fructose for sucrose synthesis (Tucker, 1993).

In tomatoes, the increased activity of acid invertase may be responsible for the low ratio of sucrose-to-hexose at maturity in commercial cultivars, whereas in wild-type tomatoes, a higher sucrose concentration could result either from the reduced activity of acid invertase (Miron & Schaffer, 1991) or high sucrose phosphate synthase (SPS) activity (Dali et al., 1992). As Nguyen-Quoc et al. (1999) showed, SPS probably affects sucrose turnover and starch synthesis but not surcrose-to-hexose ratios. However, sucrose recycling may also occur via four “futile cycles” involving sugar transport between the cytosol, vacuole, and apoplast (Nguyen-Quoc & Foyer, 2001). In these, there is continuous degradation of sucrose in the cytosol by SuSy, sucrose resynthesis via either SuSy or SPS, sucrose hydrolysis in the vacuole, or apoplast by acid invertase with subsequent transport of the hexoses to the cytosol where they are reconverted into sucrose. In this way, futile cycles of sucrose-hexose interchange govern the fruit sugar content and composition, whereas a constantly high invertase activity during the later stages of ripening maintains high cellular hexose concentrations.

In pomes, starch hydrolysis by α- and β-amylases and starch phosphorylase usually begins at the later stages of fruit growth but before the onset of the climacteric. In the ripe fruit, starch is almost totally hydrolyzed to sucrose (Berüter, 2004), which in turn is slowly hydrolyzed to fructose and glucose. Fruits that accumulate sucrose during ripening (e.g., mango, kiwi, banana, and melon) show an increase in SPS activity and a decrease in acid invertase during rapid sucrose accumulation (Hubbard et al., 1991), although gluconeogenic enzymes fructose-1-6-diphosphatase and glucose-6-phosphatase may also be involved (Cordenunsi & Lajolo, 1995).

Lipid Metabolism

Fatty acids are synthesized within the cytosol as well as within certain plastids, such as chromoplasts and chloroplasts. In avocados, saturated or polyunsaturated fatty acids are synthesized during the first weeks of fruit development whereas monounsaturated fatty acids are synthesized throughout fruit growth (Mazliak, 1970). The biosynthesis of fatty acids and triglycerides in avocado has been reviewed by Seymour and Tucker (1993).

In olives, fatty acids accumulate mainly after the period of rapid fruit growth, but later decline. Oleic acid is the principal fatty acid in olives, but palmitic, stearic, and linoleic acids are also present in significant amounts. The concentration of oleic acid increases during low temperature (Mazliak, 1970).

Palmitic, oleic, linoleic, and linolenic are the principal fatty acids formed in the peel and pulp of bananas. A decrease in fatty acid concentration occurs during fruit maturation and ripening, with linoleic acid decreasing and linolenic acid increasing (Wade & Bishop, 1978). Changes in the relative concentrations of fatty acid constituents during maturation are also seen in other fruits. For example, green pumpkins (Cucurbita pepo L.) contain linolenic and myristic acid, which are not present in ripe fruits. The lipid content of muskmelon peel does not change significantly during fruit development, but with the start of ripening unsaturated fatty acids accumulate (Forney, 1990).

Organic Acid Metabolism

Although green fruits can synthesize organic acids as products of photosynthesis, most organic acids within the fruit are derived from other parts of the plant (Ulrich, 1970), and the organic acid content may be a useful indicator of fruit maturity, for example in apples and tomatoes (Stevens et al., 1979; Kader, 1999), but varies with the cultivar and cultivation conditions. For example, the malic acid-to-citric acid ratio in tomato fruit varies between cultivar and with the stage of ripeness. Early cultivars tend to have more malate than late cultivars, whereas the citrate levels increase with ripening. Moreover, the free and total acid levels in tomatoes relate to the K concentration within the root substrate (Passam et al., 2007). Generally, overripening and aging of fruit results in a decrease in acid content and a concomitant loss of flavor.

In apples, the organic acid content increases during fruit development, but decreases before harvest and during subsequent storage. On the other hand, the organic acid content of pears decreases continually from an early stage of fruit development (Ulrich, 1970). In citrus, except lemons, acidity increases early in fruit development and then decreases during ripening (Samson, 1986), and this can be partly associated with increased fruit size and water content (Kimball, 1984). These changes mainly reflect the change in citric acid, which is the most abundant organic acid, whereas malic acid does not change significantly (Shaked & Hasdai, 1985). During storage, the organic acid content of citrus fruit decreases but the pattern of change varies with the different tissues of the fruit; for example, malate (the second most abundant organic acid of citrus) decreases in the albedo and increases in the flavedo (Sasson & Monseline, 1977). In lemons, the increase in acidity during ripening is associated with an increased concentration of citrate (Ting & Attaway, 1970).

The environmental conditions during fruit growth can significantly affect both the organic acid and the sugar content, and therefore fruit flavor and quality. Although shading did not change the citrate content of strawberries (Watson et al., 2002), the pH and reducing sugar content of peaches were higher in fruits produced under conditions of high light intensity (Geanard & Bruchou, 1992). Moreover, the afternoon exposure of fruits to the sun reduces the concentration of sucrose and malate and increases the concentration of citrate. Tomatoes produced under low relative humidity are firmer and juicier than those cultivated under high relative humidity (Janse & Schols, 1992). In tomato, too, the sugar, titratable acid, aroma volatile, and vitamin C contents are enhanced by drought stress (Auerswald et al., 1999; Veit-Köhler et al., 1999), apparently due to a combination of osmoregulation and decreased yield, which concentrates the photosynthates into fewer and smaller fruits (Beverly et al., 1993). Stress due to soil salinity can lead to an increased concentration of organic acids in tomatoes (Auerswald et al., 1999; Passam et al., 2007) and enhanced blossom end rot (Reid et al., 1996), whereas high CO2 concentrations in the greenhouse or in the storage atmosphere lead to a reduction (Huyskens-Keil & Schreiner, 2004). With the notable exception of banana, the organic acid content of fruit decreases during storage, but low storage temperatures may disrupt this process, especially in species that are sensitive to chilling injury.

Vitamin Metabolism

Vitamin A is liposoluble and is formed from carotenoids, classified as carotenes with C and H (e.g., α-carotene, β-carotene, lycopene) or xanthophylls (e.g., violaxanthin, zeaxanthin), which are oxygenated derivatives. The carotenoids are terpenoids formed by eight isoprene units that take part in radiation interception (400–500 nm) and transfer to chlorophyll during photosynthesis (Taiz & Zeiger, 2002). They are primarily responsible for the red, yellow, and orange color of fruits and also protect the photosynthetic structures from excessive energy (Grusak & DellaPenna, 1999) through a reversible reaction involving violaxanthin and zeaxanthin (Horton et al., 1996).

Vitamin C (L-ascorbic acid) is synthesized either via a pathway involving L-lactose as precursor or via the galacturonic acid pathway associated with cell wall pectin degradation. L-ascorbic acid is readily oxidized to L-dehydroascorbic acid, which can be irreversibly oxidized to form diketogulonic acid that has no vitamin C activity (Parviainen & Nyyssonen, 1992). L-ascorbic acid degradation is affected mainly by the activity of ascorbate oxidase, which is associated with rapidly growing regions of the plant.

The vitamin C content of fruit varies with genotype and environmental conditions during growth. Irradiation has a definite influence on the amount of vitamin C formed because ascorbic acid is synthesized from sugars supplied through light dependent photosynthesis (Lee & Kader, 2000). In contrast, cool temperatures increase L-ascorbic acid in citrus such as mandarins (Citrus reticulata Blanco) and grapefruits (Citrus × paradisi Macfad.) (Lee & Kader, 2000) and tomato (Islam & Khan, 2001). High rates of nitrogen application apparently reduce the L-ascorbic acid concentration in citrus, whereas potassium application or reduced irrigation may increase the L-ascorbic acid levels (Nagy, 1980; Lee & Kader, 2000). In addition, L-ascorbic acid concentrations are reduced in tissues under stress (e.g., pathogen or chemical exposure) due to increased ascorbate oxidase activity (Loewus & Loewus, 1987).

The L-ascorbic acid concentration within fruits is significantly affected by maturity. Ripe tomatoes and red peppers have a higher L-ascorbic acid content than unripe (green) ones (Howard et al., 1994). Although L-ascorbic acid accumulation in tomatoes can occur after harvest, higher levels are reached when fruits ripen on the plant. Likewise, L-ascorbic acid content is higher in apricots and peaches ripened on the plant, but apples, mangoes, and citrus have lower levels at maturity (Nagy, 1980; Lee & Kader, 2000).

The postharvest management of fruits is significant for vitamic C content. L-ascorbic acid decreases during the storage of tomatoes and apples, and losses are enhanced by mechanical injury, extended storage, high temperatures, low relative humidity, physical damage, and chilling injury (Lee & Kader, 2000). On the other hand, treatment with calcium chloride (CaCl2) can increase the vitamin C content of apples and tomatoes and the application of modified atmosphere storage reduces the rate of vitamin C loss in apples (Lee & Kader, 2000). The processing of fruits after harvest (e.g., by cooking or freezing) can also lead to significant reductions in vitamin C concentration because of the high sensitivity of L-ascorbic acid to chemical and enzymatic oxidation (Oruna-Concha et al., 1998). Similarly, vitamin E is highly susceptible to oxidation during storage and processing (Vicente et al., 2009).

Phenolic Compounds Metabolism

Phenolic compounds are synthesized in several different ways. Most phenolics are derived at least in part from phenylalanine, which is produced from the shikimic acid pathway and leads to the formation of cinnamic acid. The formation of cinnamic acid is catalyzed by phenylalanine ammonia lyase (PAL) and leads to the production of phenolic acids, such as derivatives of benzoic acid (e.g., vanillic and salicylic acids), cumaric acid and simple phenylpropanoids (e.g., caffeic, ferulic acid) (Taiz & Zeiger, 2002). One of the most common phenolic acids in fruits such as apples, pears, and peaches is chlorogenic acid, which is the dominant substrate of enzymatic oxidation and leads to cell blackening after wounding. The antioxidant capacity of the phenolic acid depends on its structure, and it is higher in molecules with a large number of hydroxyls (Vicente et al., 2009).

Fruit phenolic acid content varies with cultivar (Wang & Lin, 2000) and environmental factors during fruit development. High light intensity favors the accumulation of phenolic acids in the peel of tomato (Gautier et al., 2008), and even within the same fruit, the shaded region contains fewer phenolics than the sun-exposed region (Lee & Kader, 2000). The phenolic acid concentration of blueberry fruit decreases during ripening (Castrejon et al., 2008), as does the chlorogenic acid content of tomato, although due to fruit enlargement, the total amount of phenolic acid per fruit may increase.

Flavonoids are synthesized from products of the shikimic acid and malonic acid pathways involving the interaction of at least five different pathways: (1) the glycolytic pathway, (2) the pentose phosphate pathway, (3) the shikimate pathway that synthesizes phenylalanine, (4) the general phenylpropanoid metabolism that produces activated cinnamic acid derivatives (4-coumaroyl [CoA]) and also lignin, and (5) the diverse specific flavonoid pathways (reviewed by Robards & Antolovich, 1997). Flavonoid synthesis is affected by light intensity and wavelength, with fruits exposed to full sunlight containing more flavonoids than those in the shade (Awad et al., 2001). Moreover it can be modified by temperature, humidity, and phytoregulators, as for example in citrus (Arcas et al., 2000).

The flavonoids may affect fruit quality characteristics such as texture, color, flavor, and the nutritional value of fruits (e.g., in apples) through their involvement in the formation of undesirable brown pigments following bruising or cutting, due to the enzymatic oxidation of endogenous phenolics into quinones, which are then polymerized into brown products (Robards & Antolovich, 1997). Flavonoids increase the postharvest resistance of fruits to pathogens (Lattanzio, 2003) but may contribute to the formation of undesirable sediments in fruit juices and wine.

Anthocyanins are always produced during fruit ripening, but in some fruits such as nectarines (Prunus persica [L.] Batsch.), they are produced during the first stages of fruit development. The anthocyanin content of ripe fruit is affected by environmental conditions such as light and temperature (Faragher, 1983; Arakawa et al., 1985) and is enhanced by low rates of nitrogen fertilization. In apples, anthocyanin accumulation is favored by low temperatures, especially at night. Cyanidin is the most common anthocyanidin in fruits such as peaches and pears.

Quercetin, kaempferol, myricetin, and isorhamnetin are the most common flavonol aglycones that have been identified in plants (Robards & Antolovich, 1997). Flavan-3-ols are important constituents of fruits in oligomeric or polymeric forms, such as proanthocyanidins or condensed tannins. However, the monomers found in fruits (e.g., [+]-catechin, [2]-epicatechin, [+]-gallocatechin, and [2]-epigallocatechin) are important natural products. They are distinguished from other flavonoids because they are present usually in free rather than glycosylated forms. Moreover, catechins are the natural substrates of polyphenol oxidases and are therefore involved in browning phenomena. They are also the monomer units for procyanidins (Robards & Antolovich, 1997).

Although flavanones are generally found in small amounts in fruits, they are the predominant flavonoid of citrus (most common are naringenin, eriodictyol, isosakuranetin and hesperetin), and they are usually present in the glycoside forms. The flavone content of fruit is generally low, except citrus, where they are present as polymethoxylated flavones (e.g., nobiletin, sinensetin, and tangeretin).

In tomato, the most abundant flavonoid is naringenin chalcone. Tomatoes also contain the flavonols quercetin-rutinoside (rutin) and kaempferol-rutinoside. The accumulation of these flavonoids occurs exclusively in the peel during fruit color development (Muir et al., 2001). Cool storage leads to an increase in the anthocyanin concent of fruits such as strawberry, blueberry, and grape, but low temperatures may also have negative effects on the fruit phenolics contents (Tomás-Barberan & Espín, 2001).

Proteins and Amino Acids Metabolism

Although the protein content of fruits is low and is not considered to be a quality characteristic, proteins nevertheless play an important role as enzymes in the physiological processes of ripening and senescence before and after harvest. Thus, fruit development and ripening are related with quantitative and qualitative changes in protein synthesis that vary between varieties and with the season of cultivation. Moreover, amino acids can serve as precursors for lipid, carbohydrate, and nucleic acid synthesis.

Amino acid metabolism is complex because of the large number of metabolites involved (Buchanan et al., 2000) and may follow different pathways according to the different length of carbon structures. For example, alanine is degraded to pyruvate and can be utilized in gluconeogenesis. Aspartate and asparagines are degraded to oxaloacetate and are closely linked to glutamate and α-ketoglutarate interconversion by amino transferases. Glutamine, proline, and arginine are converted ultimately to glutamate, which is deaminated to α-ketoglutarate. Other nonpolar amino acids, such as methionine and valine, are precursors for the synthesis of odd numbered fatty acids via the intermediate propionyl-CoA.

Free amino acids (e.g., aspartic acid, asparagine, proline, lysine, α- και β- alanine) are present in fruits and especially in their juices. The concentration of free amino acids alters during fruit development and ripening depending on the species (Burroughs, 1970). The onset of ripening of climacteric fruits is also dependent on amino acid metabolism because of the involvement of L-methionine in ethylene synthesis.

In ‘Valencia’ oranges, proline amounts to as much as 50% of the total amino acid concentration, whereas serine and glutamic acid constitute about 10%. The amino acid content varies with the stage of ripening (Tadeo et al., 1988) and is affected by temperature. In grapes, glutamic acid and arginine are the principal amino acids in the juice. Their concentration depends on the cultivar, rootstock, degree of fruit maturity, temperature, and mineral nutrition. The total amino acid and total free amino acid content of grapes increase during maturation, and synthesis occurs mainly during the last 6 to 8 weeks of fruit ripening (Kanellis & Roubelakis-Angelakis, 1993). In cherry, proline and asparagine accumulate in the red fruit, whereas in tomato, glutamic acid and aspartic acid increase during fruit ripening, especially following the removal of fruit from the plant, and other amino acids (e.g., arginine and alanine) decline. High levels of nitrogen or phosphate application increase the total amino acid content of tomatoes, but high levels of potassium lead to decreased levels of total amino acids (Saravacos et al., 1958).

Mineral Metabolism

The mineral content of fruits greatly contributes to their quality characteristics and nutritional value and may be affected by several factors, such as environment (i.e., temperature, humidity, sunlight) and cultural practices (e.g., organic culture, hydroponic culture). For example, tomatoes grown organically in a compost/soil mix had a higher calcium and lower potassium, magnesium, and sodium concentration than those grown hydroponically (Premuzic et al., 1998).

Nitrogen is a component of free amino acids (about 80% of the total nitrogen content) and can be found in nonprotein nitrogenous combinations (e.g., choline, glutathione, asparagine, purine). The presence of nitrogen within the fruits varies from tissue to tissue because tissues with high metabolic rates, such as the epicarp and core, may have higher requirements for nitrogen (Vicente et al., 2009). High amounts of nitrogen in apples favor the development of green color (Marsh et al., 1996), whereas excess nitrogen in peaches lead to an inhibition of color change from green to yellow (Crisosto et al., 1997) as well as color development in citrus and grapes.