Post harvest biology and technology of citrus fruits is gaining importance as the therapeutic value of citrus fruits is realized and supported by the increase in health awareness among the general public This book is the most comprehensive reference on citrus fruit biology, biotechnology and quality Basic and applied scientific information is interwoven to serve the researcher, marketer, scientist, nutritionist, or dietician With discussions of fruit morphology, anatomy, physiology and biochemistry and chapters on growth phases, maturity standards, grades and physical and mechanical characteristics of citrus trees, this book provides the foundation for understanding growth, harvest and post harvest aspects of these important plants Insect-pests and diseases, irrigation, nutrition and rootstocks are also addressed * Provides practical tips for post harvest management * Includes all aspects of citrus fruit biology, technology and quality evaluation * Discusses biotechnological applications and potential fresh citrus fruit quality improvement * Evaluates medicinal and therapeutic applications and recent clinical findings * Exhaustive glossary included

Citrus Fruit: Biology, Technology and Evaluation

Citrus is the main fruit tree crop in the world and therefore has a tremendous economical, social and cultural impact in our society. In recent years, our knowledge on plant reproductive biology has increased considerably mostly because of the work developed in model plants. However, the information generated in these species cannot always be applied to citrus, predominantly because citrus is a perennial tree crop that exhibits a very peculiar and unusual reproductive biology. Regulation of fruit growth and development in citrus is an intricate phenomenon depending upon many internal and external factors that may operate both sequentially and simultaneously. The elements and mechanisms whereby endogenous and environmental stimuli affect fruit growth are being interpreted and this knowledge may help to provide tools that allow optimizing production and fruit with enhanced nutritional value, the ultimate goal of the Citrus Industry. This article will review the progress that has taken place in the physiology of citrus fruiting during recent years and present the current status of major research topics in this area.

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Physiology of citrus fruiting

ADDITIONAL INDEX WORDS. citrus, cytoplasmic male sterility, seedless, parthenocarpy, mutation breeding, ploidy ABSTRACT. Seedlessness can be obtained through parthenocarpy (i.e., fruit formation without fertilization or embryo abortion). In practice, the actual reduction in seed number in parthenocarpic plants is often exaggerated by coupling parthenocarpywithself-incompatibilityormalesterility.Traitsrelatedtoseedlessness, suchasparthenocarpy,canbe introduced into genetic accessions through conventional cross-breeding. However, conventional breeding in Citrus L. species faces several limitations. First, these species have long juvenile (nonflowering) periods during which a relatively thick canopy develops, which limits the size of seedling populations that can be maintained for further evaluation. Second, they have a narrow genetic base, which limits the availability of alternative alleles that could be introgressed into other lines for the formation of a particular phenotype such as parthenocarpy. Third, breeding efforts are limited by the lack of knowledge of the mode of inheritance of specific characteristics. Fourth, and last, breeding efforts are limited by the polygenic nature of many important traits. Despite these limitations, conventional breeding in fruit trees has yielded improved cultivars and will most likely continue to be a very important strategy. However, emerging biotechnological approaches should be continuously evaluated for their potential for expediting such breeding efforts. The objective of this review is to present, evaluate, and discuss conventional and emerging biotechnological approaches for the induction and maintenance of seedlessness in a variety of crops. Particular attention will be paid to citrus crops, including the presentation and discussion of some preliminary data on the genetic inheritance of parthenocarpy. Citrus is a common term for a genus (Citrus) of flowering plants in the family Rutaceae, documented as early as the ChineseBook of History, dated 1000 BCE. Citrus fruit are valued for their vivid colors; strong fragrances; sharp, delicious tastes; and high nutritional value. Today, citrus ranks first in world fruit production (Food and Agriculture Organization of the United Nations (FAO), 2005). It seems likely that the center of origin of commercially important citrus species lies in southeast Asia; many of these species probably originated in China. From the proposed dis- persal history of ancestral taxa, it seems that although orange (Citrus sinensis (L.) Osbeck.), lemon (Citrus limon (L.) Burm. f.), mandarin (Citrus reticulata Blanco), pummelo (Citrus grandis Osbeck), and grapefruit (Citrus paradisi Macf.) are now distributed worldwide, they share a relatively narrow genetic base. All were initially selected by humans and later propagated through apomictic embryos, cuttings, or grafting. Nearly all the variation in the polyembryonic cultivars appears to be the result of apomixis and somatic mutations. For centuries, some zygotic seedlings have been selected and propagated, such as the 'Clementine' mandarin and grapefruit. Seeded grape (Vitis vinifera L.), citrus, and watermelon (Citrullus lanatus Thunb.) were long accepted by consumers. However, during the past few decades, consumer interest in seedless fruit has increased, and seedlessness has become an important characteristic for fresh market fruit, including citrus. Parthenocarpy

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Induction of Seedlessness in Citrus: From Classical Techniques to Emerging Biotechnological Approaches

Reproductive and vegetative tissues of the seeded Pineapple cultivars of sweet orange (Citrus sinensis L.) contained the following C-13 hydroxylated gibberellins (GAs): GA53, GA17, GA19, GA20, GA1, GA29, and GA8, as well as GA97, 3-epi-GA1, and several uncharacterized GAs. The inclusion of 3-epi-GA1 as an endogenous substance was based on measurements of the isomerization rates of previously added [2H2]GA1. Pollination enhanced amounts of GA19, GA20, GA29, and GA8 in developing ovaries. Levels of GA1 increased from 5.0 to 9.5 ng/g dry weight during anthesis and were reduced thereafter. The amount of GA in mature pollen was very low. Emasculation reduced GA levels and caused a rapid 100% ovary abscission. This effect was partially counteracted by either pollination or application of GA3. In pollinated ovaries, repeated paclobutrazol applications decreased the amount of GA and increased ovary abscission, although the pattern of continuous decline was different from the sudden abscission induced by emasculation. The above results indicate that, in citrus, pollination increases GA levels and reduces ovary abscission and that the presence of exogenous GA3 in unpollinated ovaries also suppresses abscission. Evidence is also presented that pollination and GAs do not, as is generally assumed, suppress ovary abscission through the reactivation of cell division.

Pollination Increases Gibberellin Levels in Developing Ovaries of Seeded Varieties of Citrus

Satsuma [Citrus unshiu (Mak) Marc.] and Clementine [Citrus reticulata (Hort.) Ex. Tanaka, cv. Oroval] are two related species of seedless mandarins which differ in their tendency to set parthenocarpic fruits. Satsuma fruits naturally set parthenocarpically whereas Clementine mandarins show very low ability to set fruit in the absence of cross-pollination. The endogenous levels of gibberellins (GAs) and free and conjugated indole-acetic acid (IAA) and abscisic acid (ABA) throughout early stages of fruit development were investigated in seedless cultivars of both species. Analyses performed by full-scan combined gas chromatography-mass spectrometry (GC-MS) of extracts from ovaries at anthesis demonstrated the presence of GA19, GA20, GA29, GA1, GA8, GA3 and iso-GA3 in Satsuma mandarin, whereas only GA29, GA3 and trace levels of GA8 were detected in Clementine. At this developmental stage GA-like substances, as estimated by bioassay, reached their highest levels in Satsuma, while Clementine mandarins contained relatively lower levels. In both species the highest levels of free IAA were found at petal-fall stage at which time free ABA levels also peaked. Developing fruits of Clementine had higher amounts of both free IAA and ABA. In Satsuma, levels of conjugated IAA remained low throughout reproductive development whereas in Clementine they increased as the free form declined. In contrast, conjugated ABA was at low levels in Clementine but reached higher concentrations in Satsuma. These results suggest that in these mandarins the potential for setting parthenocarpic fruits is mainly influenced by the hormonal status of the fruit during the later stages of cell division and early stages of cell enlargement. Thus, the condition of low ability to set parthenocarpic fruits appears to be associated with lower levels of active GAs, lower capability to catabolize ABA to conjugated ABA and higher ability to conjugate IAA during this period.

J.L. Garcia-Martinez

Papers

Endogenous plant growth substances content in young fruits of seeded and seedless Clementine mandarin as related to fruit set and development

Influence of gibberellic-acid on early fruit-development, diffusible growth substances and content of macronutrients in seedless clementine mandarin

Fruit-set and development in seeded and seedless clementine mandarin