Dr. Richard E. Litz, Tropical Research and Education Center / Center for Tropical Agriculture, University of Florida, USA, (relitz@ufl.edu), April 2009.
Civilization, culture and fruit
Many fruit tree selections are ancient representatives of mankind’s patrimony and larger cultural story. Our history and culture have been impacted in many ways by our love of fresh fruit and their products. In some manner, the major fruits have entered into our consciousness, and have affected our view of the world. For example, the ancient Mediterranean civilizations were well acquainted with the grape (Vitis vinifera) and the effects of wine consumption on unleashing the human spirit. Dionysius was god not only of wine but also of the arts. The Greek word Συμπόσιον “symposium” originally referred to “drinking (wine) together.” European empires have come and gone during the last two millennia; however, the lasting heritage of grapes has been extensively documented. In the De Re Rustica, a 12 volume encyclopedia of Roman agriculture written by Lucius Iunius Moderatus Columella (4 – c 70 AD), there is a description of the grape cultivar ‘Duras’, which is still cultivated for wine production in parts of Europe.
This cultural affinity with fruit also holds true for the tropics. The Gautama Buddha (563 – 483 BC) is said to have been born beneath either a mango (Mangifera indica) or a sal (Shorea sp.) tree in Lumbini (India). The enchanting Amrapali, a dancer and a courtesan of the city of Vaishali, was so moved by his message that she presented a mango orchard to the Buddha; the ‘Amrapali’ is one of India’s great mango cultivars. An interesting miracle performed by the Buddha is said to have involved the king’s gardener, Ganda, who presented a ripe mango fruit to Buddha. The Buddha then told Ganda to plant the seed, from which immediately sprang a large fruit-bearing tree.
any mango cultivars that we know today were selected at the time of the Moghul emperor Akhbar (1556-1605), and were grown in the Lakh Bagh, a large orchard of 100,000 trees, near Darbhanga in Bihar state. The Ain-i-Akbari, an encyclopedic work that was written by Akbar’s friend and vizier, Abul-Fazl ibn Mubarak, contains a lengthy account of the mango.
Fruit and the evolution of development (evo devo)
Fruit trees are also repositories of invaluable information about the evolutionary history of higher plants. The avocado (Persea americana), for example, is in the family Lauraceae, which is part of a basal clade that contains most of the primitive angiosperms, originally classified as the magnoliids. The magnoliids include the Magnoliales, e.g., magnolia Magnolia spp., and the Laurales, e.g., avocado, in one clade and the Piperales, e.g., black pepper Piper nigrum, and Canellales, e.g., cinnamon Cinnamomum verum, in another clade. Many notable trends in floral evolution can be demonstrated with flowers of the Laurales: floral phyllotaxy (spiral to whorled), fusion among floral parts and variations in the number of floral organs. MADS box gene expression in avocado flowers differs from the ABC(DE) model, Arabidopsis thaliana, and may have evolved from a regulatory network inherited from stamens. Avocado flowers therefore provide a unique opportunity to examine the changes in regulatory controls that necessarily accompany this avenue of perianth evolution. Fruit development, maturation and ripening in avocado also depart significantly from the model plants. The great antiquity of the avocado suggests that evolution of flower and fruit development can be studied in the context of a link that joins the most primitive angiosperms with non-flowering ancestors, i.e., the gymnosperms. Hitoshi Kihara, the great Japanese plant geneticist eloquently stated: “The history of the earth is recorded in the layers of its crust. The history of all organisms is inscribed in the chromosomes.”
Fruit and genetic improvement
Kihara wrote a few years before the green revolution, which was based on classical breeding and genetics, transformed food production. This revolution, however, has had little impact on the improvement of most fruit trees, which even today are likely to have been chance seedling trees produced from uncontrolled pollinations. The long juvenile period of trees is probably the most widely accepted explanation for this anomaly; however, cultural issues must not be overlooked. In many societies, as described previously, there are strong cultural relationships with certain fruit selections. This is particularly true in south and south-east Asian cultures, where consumers are highly resistant to changes in the traditional diets. These traditional fruit selections often have serious production problems. 1) They lack resistance to many diseases and abiotic stresses. 2) Insect pests may be attracted to certain selections. 3) They have low yields and 4) they may be alternate bearing. 5) The tree architecture may not be suited to efficient harvesting and management. 6) The fruit may have poor shelf life. 7) The fruit quality (taste, texture, size, aroma, etc.) may be poor.
These and other problems afflicting particular cultivars have a genetic basis and they cannot be addressed by advances in horticultural practices and technologies alone. Is conventional breeding the answer? Certainly, genomics and marker-assisted breeding can accelerate breeding strategies of perennial fruit trees; however, these technologies are currently being developed only for a handful of crop species: citrus, fruits in the Rosaceae [apple Malus x domestica), peach (Prunus persica), apricot (P. armeniaca), plum (P. domestica), cherry (P. avium), cacao (Theobroma cacao) and papaya (Carica papaya)]. Even with these examples, molecular tools for screening large populations of seedlings that have been derived from controlled populations is still very much in the future: very few markers have been identified. Genetic engineering also must have a role in the development of superior lines of existing fruit cultivars. The first wave of genetic engineering was based upon induced mutagenesis, and had moderate success with perennial fruit crops in the early years; however, the random generation of point mutations, many of which were deleterious, seriously impeded the continued use of this technology. Mutation breeding has largely been superseded by genetic transformation. This technology, although widely accepted in the major food exporting (USA, Brazil, Argentina, Australia, Canada) and underdeveloped and developing countries, remains controversial in the EU, Japan as well as with many consumers. In many ways, genomics and genetic transformation represent the direction that fruit breeding must take in the future. To date, only two fruit crops have received clearance for human consumption, papaya and plum, both of which were being decimated by virus diseases caused by papaya ringspot virus and plum pox virus, respectively, with no resistance to the diseases in the ‘Solo’ type of papaya and in plum. Papaya and plum were engineered to be resistant to these virus diseases. In effect, genetic transformation must be credited with saving both of these crops. In the future, as the tools of genomics identify more genes that mediate horticultural traits, we anticipate that the fruit cultivars that have been treasured for many centuries can be modified to correct or reduce the effects of major production and post harvest challenges.
Bibliography
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- Gonsalves, D. 1998. Control of papaya ringspot virus in papaya: a case study. Ann. Phytopath. 36: 415-437.
- Litz, R. E., M. A. Gomez Lim and U. Lavi. 2009. Biotechnology. In: The Mango and Its Botany, Production and Uses, 2nd Edition, ed. by R. E. Litz, CAB International, Oxon, pp. 641-669.
- Maluszynski, M. 2004. Officially Released Mutant Varieties – The FAO/IAEA Database. Plant Cell Tiss. Org. Cult. 65: 175-177.
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