The true citrus fruit trees belong to the family of Rutaceae, subfamily Aurantioideae. Rutaceae is one of the four families in Rutales, division Lignosae of the subphylum Dicotyledoneae, with mostly subtropical or tropical genera. Leaves usually possess transparent oil glands and flowers contain an annular disc. Rutaceae contains about 150 genera and 1600 species (Swingle and Reece, 1967).

Aurantioideae, the ‘Orange’ subfamily – one of seven subfamilies in Rutaceae (Engler, 1931) – has been subdivided by W. T. Swingle into two tribes–Clauseneae and Citreae (a single tribe according to Engler, 1896), with 33 genera and 203 species. The main characteristics of the subfamily are: the fruit is a berry (hesperidium) with a leathery rind or hard shell, often with juicy pulp in the subtribe Citrineae. The seeds are without endosperm, sometimes with two or more nucellar (apomictic) embryos. The leaves and bark have schizolysigenous oil glands. They are small or sometimes large trees, rarely shrubs. Incorporating new taxonomic information, the number of species is now estimated at about 220, though a reduction in according species rank to several citrus species will reduce this number (see Table 2.1). Relationships between genera have in the past been based mainly on comparative morphology. Morphological affinities have been supported to some extent by grafting and hybridization performance.

Citrus relatives

An increase in interest in the Aurantioideae has been shown in the quest for wild members of the subfamily as a source of novel genetic variation and as a possible source of rootstocks, as well as for traits for pest and disease resistance.

The biology, history and development of citrus fruits have aroused worldwide interest. This has been enhanced, to a large degree, by the uniquely attractive appearance of the fruit and by its medicinal properties.

Citrus fruits originated in South East Asia and spread during the Middle Ages, later to become established in all continents. Citrus is by far the most important evergreen fruit crop in world trade. The fruit's special structure and long shelf life have facilitated its large-scale export as fresh fruit. Processed juice products, on the other hand, have also become increasingly important worldwide.

The exact origin of Citrus, its ancestral types and systematics are still largely unknown. The great wealth of citrus types and cultivars of today reflects the vast natural breeding options within Citrus, as well as effective intentional human intervention. Molecular genetics, which has been a most helpful tool in unraveling the secrets of the past, also opens new vistas for breeding work in the future. Modern citriculture has adopted parthenocarpy and seedlessness for all major citrus types. Present-day cultivars represent largely subtle gene combinations conserved by vegetative propagation on seed-propagated, apomictic rootstocks.

Classical citriculture achieved the highest fruit quality in subtropical areas. Low temperatures and frost hazards limit the expansion of citriculture into cooler domains. Citrus has always been known to be highly dependent on irrigation in most environments.

Introduction

For a comprehensive description of the vegetative Citrus tree it is not enough to provide a structural description of the basic organ units. Attention must also be paid to physiological activity, at the organ level as well as at the whole-plant level. Environmental, eco-physiological aspects of tree activity must also be considered if a broader understanding of citrus tree behavior is to be achieved.

Citrus trees belong to the ‘evergreens’, which do not shed their leaves during the fall. The evergreen habit has important consequences for leaf longevity and physiological activity, which must be reflected in leaf design and structure. In the absence of fall abscission the longevity of leaves may extend to a whole year and beyond. The year-round presence of leaves enables uninterrupted, day-by-day photosynthetic activity, and thus a continuous supply of photosynthates. However, in subtropical climate zones winter temperatures might be quite low so that photosynthetic gains during winter are lowered. The evergreen habit may have significant implications with regard to the role of nutrient reserves, particularly during springtime. Whereas deciduous trees are totally dependent for early spring growth upon their carbohydrate reserves, evergreens like citrus may at least partly rely on the supply of photosynthate from the previous season's foliage. The evergreen character also has far-reaching consequences for the annual cycle of flowering and fruiting, as will be discussed in Chapter 4.

History of citrus

The term citrus originated from the Latin form of ‘Kedros’, a Greek word denoting trees like cedar, pine and cypress. As the smell of citrus leaves and fruit was reminiscent of that of cedar, the name citrus has been applied to the citron. Linnaeus grouped all citrus species known to him in the genus Citrus. In Greek mythology citrus fruits are called hesperides.

The suggested origin of the true citrus fruits is South East Asia, including South China, north-eastern India and Burma. Evidence from wild citrus in the area is still unclear. In many cases, seed has been spread large distances from the sites of origin and culture by birds, water streams and human activity. Tolkowsky (1938) considers the centre of origin to be the mountainous parts of southern China and north-eastern India, where sheltered valleys and southern slopes are protected from cold and dry winds yet are exposed to the warm rains of the summer monsoon. The deciduous Poncirus trifoliata grows wild in central and northern China.

While according to certain authors (Tanaka, 1954; Jackson, 1991) citrus fruits may have originated in north-eastern India and Burma, the introduction of citrus into cultivation and the probable origin of several species started in China. Table 2.2 gives the principal species of Citrus and their probable native habitat, according to Cooper and Chapot (1977). Lemon and grapefruit are not considered true species.

The flowering of citrus

Introduction

The transition of the vegetative, leaf producing meristem into the reproductive floral meristem is the initial event in the long chain of developmental processes leading to seed and fruit production.

The environmental and endogenous control of flower bud differentiation is quite complex and varies considerably from one species to another. Citrus trees, like other fruit trees, are polycarpic plants undergoing repeated cycles of flowering and fruiting. Fruit trees never commit all their buds to flowering – a certain number of buds must be retained in the vegetative, non-differentiated state to ensure the tree's future. This raises certain questions with regard to the nature of floral induction in fruit trees. The suggestion has been made that flowering in fruit trees might be under ‘negative’ control, i.e. all buds are induced to flower but their actual flowering is controlled by the presence of a ‘flowering inhibitor’. This hypothesis must be weighed against the more common concept of a positive ‘floral stimulus’ which must reach the apex to start the process of flower bud differentiation (Lang, 1965).

Certain aspects of citrus' floral development derive from the nature of citrus as a tropical-subtropical evergreen which, unlike deciduous fruit trees, does not have true dormancy (Monselise, 1985). Deciduous fruit trees form flower buds during early summer. These flower buds complete their morphological development prior to the onset of winter dormancy and appear to be ready by the fall for the burst and bloom of the following spring.

The present book aims to provide a concise, up-to-date reference book on most aspects of citrus biology. Citrus is second only to the grape (the largest area of which is planted for wine) as a fruit crop and has been the subject of many studies. Six volumes of Citrus Industry issued by the University of California, dealing in detail with many aspects of citrus, have been published since 1967. Important information can be gathered also from the Proceedings of International Congresses of Citriculture. Our book provides an introduction to and overview of underlying principles and findings of citrus biology and culture. However, some important topics may have been omitted or treated too briefly. Emphasis has been placed on up-to-date treatment and conceptions of citrus physiology, reproductive development, taxonomy, genetics and breeding. The extensive references accompanying each chapter, including recommended reading, will be helpful to the reader, though they are of course, far from complete. Illustrations have been provided throughout to accompany the text.

Certain prominent up-to-date aspects of cultivated citrus are contained in a separate chapter devoted to the subject. The book also contains a contribution by Prof. D. Rosen, of the Faculty of Agriculture, the Hebrew University, on citrus pests, which has been specifically written for this book. The book will be useful to undergraduates, as well as to students in advanced courses, specializing in citriculture and horticulture. An understanding of elementary plant sciences is being assumed.

Orchard design and spacing

With careful adjustment of rootstocks and cultural practices citrus can be grown satisfactorily on a wide range of soils. In general, the deep, well-drained sandy loam soils are best suited for citrus production. No single characteristic of good citrus soil is more essential than good drainage. Without satisfactory drainage, accumulation of free water in the root zone results in poor aeration and injury to roots. In regions of heavy rainfall, the use of shallow soils with impervious subsoils or with hardpan is particularly hazardous, because under such conditions roots are most susceptible to fungal infection. Lack of drainage also contributes to effects caused by salinity which, in turn, may reduce yields. As salinity of irrigation water increases, it is necessary to move more water through the root zone to carry out accumulated salts. Thus, any restriction of drainage becomes especially harmful where irrigation is practiced.

In detailed planning of the orchard, decisions regarding planting distances and tree spacing are most important. For many years, considerations have centered on the distance which would optimize yields during the entire life of the orchard (several decades), also allowing sufficient space to conduct necessary cultural operations. Trees have been planted at distances assuring adequate light for the tree and the passage of equipment at maturity. Thus, distances between trees (varying, of course, with variety, rootstock, etc.), have generally ranged 6 to 10.5 meters in either direction, resulting in tree densities of 86–270 trees per hectare in California, Florida and South Africa.

Introduction

Most citrus cultivars grown arose as chance seedlings or bud mutations of existing cultivars. A relatively small number of cultivars of widespread significance have so far originated from breeding programs. The genetics and breeding of citrus were reviewed by Cameron and Frost (1967), Cameron and Soost (1969), Soost and Cameron (1975), Vardi and Spiegel-Roy (1978) and more recently by Soost (1987) and Gmitter et al. (1992). Citrus cultivars are highly heterozygous (Soost and Cameron 1975). Little information has been obtained on the genetic control of traits. Leading cultivars represent subtle gene combinations, often of a highly elevated ‘selection plateau’. Such combinations are disrupted by the sexual process. Many traits are polygenic as to their inheritance, being controlled by numerous genes. The probability of recombining genes in a successful hybrid to recreate the essential characters of a leading traditional cultivar is very low. Citrus breeding is also much hampered by the highly pronounced juvenility in both sexual and nucellar citrus seedlings. A further significant barrier to citrus hybridization and easy transfer of genetic material is the widely encountered apomixis (nucellar embryony) (Frost and Soost, 1968). The ability to cross within or between species in which few or no monoembryonic taxa are available – as in the orange and grapefruit – is highly restricted.

The situation has been somewhat alleviated by the increase in the list of monoembryonic cultivars produced by breeding; this may improve further by the addition of products of somatic hybridization as a result of protoplast fusion.

Introduction

Climate has been of great importance in the development of associations between trees and pathogenic fungi. In particular, the geographic range of each species of tree or fungus is delimited by factors such as temperature, moisture, snowfall and windiness which affect growth, reproduction and dispersal. Such factors affect the incidence of diseases by determining the distribution of a particular pathogen in relation to the geographic range of a potential host. Also, within a region where both host and pathogen are present, the severity of disease can vary with climate. Such variations can result from the direct effects of climatic factors on the pathogen, or from their effects on aspects of host physiology which determine resistance to attack. Other effects may involve other organisms with which either the host or pathogen interact.

In natural ecosystems, associations between particular tree and fungal species are often of great antiquity and have evolved in ways which tend to avoid mutual destruction. Environmental stability may have been a prerequisite for the development of many of these host-pathogen associations and, if that is the case, it follows that they will be perturbed by major climate change. Less stable relationships tend to occur in the simpler ecosystems that initially exist in man-made plantations, often involving new combinations of host and pathogen species that have artificially been transported beyond their natural geographic ranges.

Introduction

Recently there has been a considerable increase in interest in developing genetically modified microorganisms (GMMs) for a range of different purposes in the environment such as bioremediation, mineral leaching, improvement of soil nutrient status and biological control. Numerous experiments, virtually all with bacteria, have been carried out using genetic manipulation to increase or decrease expression of genes associated with these processes or to transfer or delete them (see Crawford et al., 1993; Stotzky et al., 1993; Lindow, Panopoulos & MacFarland, 1989). However, before commercial development of these functional GMMs can occur, the environmental risks associated with the release into the environment of GMMs lacking deliberate functional manipulation must be addressed. Consequently, for the purpose of estimating the risks of such GMM releases, a series of step-wise assessment procedures has been developed. This approach was recommended by the Organisation for Economic Co-operation and Development (OECD, 1992) and has been adopted in regulatory frameworks in most countries. These start with laboratory and glasshouse studies under contained conditions and, depending on the results, are followed by field releases and post-release monitoring procedures.

Terminology associated with risk assessment is not strictly defined (see Teng & Yang, 1993), but essentially the key initial process in the risk assessment procedure is to identify any potential hazards associated with the use of a GMM. These include pathogenicity, phenotypic and genetic stability of the parental strain, potential for survival, establishment and dissemination, potential for gene transfer and, finally, potential to affect or cause an impact on other organisms or ecological processes.

Much effort has been made in the field of plant disease epidemiology to link disease incidence and severity with short-term weather variables. However, few studies have been published on the effects of longer-term weather patterns or climate change on plant disease, despite an earlier presentation of a good case for this type of approach (Coakley, 1988). The Fusarium foot-rot disease complex of wheat provides an interesting opportunity for the study of the impacts of climate on both the severity of a disease of an herbaceous plant and the competition between the various pathogen species capable of causing foot-rot symptoms. This chapter outlines a recent study on the effects of climate, particularly air temperature, on Fusarium foot rot in UK cereals, with the aim of developing predictive models.

The disease

Fusarium foot rot of temperate cereals is a disease caused by several Fusarium or Fusarium-like species, each of which may infect stem bases individually or, in complex infections, involving two or more species. Fusarium foot rot in winter wheat is commonly caused in the UK by four species: Fusarium culmorum, Fusarium avenaceum, Microdochium nivale (formerly Fusarium nivale) and to a lesser extent F. graminearum (Parry et al., 1994). The foot-rot symptoms caused by the four species are indistinguishable (Fig. 2.1) and currently, the individual species present can only be reliably identified by the presence of perithecia in infected material or by isolations from diseased tissue pieces, using selective agar media and identification of emerging fungal colonies.

Introduction

Following the Chernobyl nuclear reactor accident in 1986, there has been much scientific effort to establish the extent of radiocaesium contamination of the terrestrial environment, and to determine the propensity for radiocaesium to transfer to plants and through the food chain to man. In upland ecosystems (grassland and forest) fungi play a major role in controlling cycling of nutrients in soil and, through mycorrhizal associations, the uptake of nutrients into plants. Little attention, however, has been directed at understanding the role of fungi in the movement and availability of Cs in soil.

It has been shown that radiocaesium may be accumulated in the basidiomes of basidiomycete fungi (Haselwandter, 1978; Eckl, Hofmann & Turk, 1986; Elstner et al., 1987; Byrne, 1988; Dighton & Horrill, 1988; Haselwandter, Berreck & Brunner, 1988; Oolberkkink & Kuyper, 1989; Watling et al., 1993). Post-Chernobyl levels of radiocaesium in fruiting structures range from background to 15 000 Bq kg-1 dry wt depending on author, fungal species and locality. Ectomycorrhizal basidiomycete species investigated by Byrne (1988) contained a range from background to 117 Bq kg-1 dry wt of total radiocaesium; Dighton and Horrill (1988) recorded a range of 3890–15 820 Bq kg-1 dry wt for the basidiomes of the ectomycorrhizal species Lactarius rufus and Inocybe longicystis. The more recent study of Watling et al. (1993) showed 137Cs contents of fruiting structures from a range of fungal species from different habitats in the UK.

Introduction

The soil environment is complex and infinitely variable (Metting, 1992). The fate of metal contaminants is similarly diverse and dependent on many factors, such as mineral composition and organic content (Krosshavn, Steinnes & Varskog, 1993), and is mediated by physicochemical processes (Zhu & Alva, 1993). Metal contaminants in the soil undergo complex interactions with both organic and inorganic components, and many studies have shown that the basic inorganic aluminosilicate clays, silts, sands and other mineral components are important metal sequestrants (Farrah & Pickering, 1976, 1977; Kuo & Baker, 1980; Harter, 1983; Krosshavn et al., 1993). It is important to recognize, however, that the soil organic component, which contains both living organisms and their decay or metabolic products, also exerts a strong influence on metal retention.

Fungi exist in soils primarily as saprotrophic degraders of organic matter, and also as pathogens of plants and in mycorrhizal associations with plant root systems. Fungi are ubiquitous soil microorganisms, predominant in acidic soils, often comprising the largest pool of biomass (including bacteria, microalgae, actinomycetes, protozoa, nematodes, earthworms and other invertebrates) and organic products under these conditions (Metting, 1992). This, combined with their high surface area to mass ratio, ensures that fungal–metal interactions are of primary importance in the organic soil environment (see Colpaert & Van Tichelin, Chapter 9). This is especially true of acidic soil conditions where metals are more likely to be speciated into soluble and more mobile forms (Hughes & Poole, 1991) and where metal ion/fungal/mineral interactions are more likely to occur due to the predominance of the fungal component of the biota in such soils.

Introduction

Microorganisms, including fungi, are known to accumulate metals from their external environment and the possibility of using fungi as a means of treating metal/radionuclide-containing effluents is well recognized (Siegel, Galun & Siegel, 1990; Gadd, 1993). However, to date, there are no commercial systems in operation which specifically use fungi as a basis for a metal treatment system. This is despite the fact that certain fungal species, under optimal conditions, are as effective as ion exchange resins in the removal of metals from solution (Tsezos & Volesky, 1981). As yet, the development of this potential from scientific curiosity to commercial fact remains to be demonstrated.

The mechanisms of microbial metal uptake may be either independent of, or dependent on, cell metabolism (Huang, Huang & Morehart, 1990; Avery & Tobin, 1992). Metal uptake which is independent of cell metabolism will be referred to as biosorption in this work. It is generally regarded that biosorptive metal uptake mechanisms would be more appropriate for use in a metal treatment system (Kuyucak, 1990). This is because environmental conditions in most effluents may be too toxic for microbial growth. Biosorption, in many cases, accounts for most of the metal accumulated by the cell, and can represent 10–20% or more of the cell dry weight (Luef, Prey & Kubicek, 1991; Gadd, 1993). The process occurs by either physical or chemical means and usually involves surface interactions of metals with microbial cell walls or excreted cell products.