Eduardo CesarSince last month, the search for orange trees that are more resistant to the main diseases that devastate Brazilian orange growing can count on a network of new allies. In a work that literally took root in six localities of the southeastern region, agronomists and technicians from the Sylvio Moreira Citriculture Center – a research unit linked to the Campinas Agronomic Institute (IAC), located in the city of Cordeirópolis, São Paulo – finished, in January, bedding out the last seedlings that make up a set of 12,000 very special plants. In these experimental orchards, located on land belonging to four municipalities in São Paulo (Araraquara, Botucatu, Itapetininga and Cordeirópolis), one in Minas Gerais (Comendador Gomes) and one in Paraná (Maringá), there are samples of 751 hybrids recently developed by researchers from the center.
Each new variety carries some characteristic that apparently confers on it more resistance, or even immunity, to diseases like leprosis, gummosis, citrus canker, and citrus variegated chlorosis (CVC), this latter commonly known as the yellowing disease. Almost 65% of the plants are hybrids in the crown, the upper part of the tree, responsible for the look and the taste of the fruit. In the other 35%, the quality of hybridism is present in the root system, in the so-called rootstock.
“The crowns and the rootstocks most used in our orange groves show a low genetic diversity”, says Marcos Machado, from the citriculture center, the coordinator of the researches with the hybrids, carried out in the ambit of the Citros Genome Project, one of the Millennium Institutes funded by the National Council for Scientific and Technological Development (CNPq). The situation of the Rangpur lime, the rootstock most disseminated in São Paulo citriculture, is emblematic: all its seedlings derive from a single clone. They have, therefore, rigorously the same genome. “We have to broaden the genetic base in the orange tree plantations, for us to cease to be the paradise for diseases”, Machado explains.
All the 751 hybrids underwent a period of preliminary tests in plant houses, a more controlled environment than the open grove to which they were transferred. Only those that proved to be more promising ended up being selected to make up the experimental network set up by the center. With luck, in two or three years, the researchers hope to have to hand the scientific proof that some of these hybrids really are resistant to the main citrus diseases.
In the last stage of the selection process, a few varieties of citrus should be left over with a potential for being commercially exploited. This is because, besides showing that they are more tolerant to diseases, the hybrids will have to preserve the characteristics of flavor associated with the Brazilian orange and show good productivity from the economic point of view. “If we get two good hybrids at the end of the project, the work will then have been worthwhile”, is Machado’s opinion.
In most cases, the hybrids were obtained by crossbreeding two citrus species with diametrically opposed characteristics: one of them would show great susceptibility to one or several diseases, while the other would prove to be more resistant to or tolerant of these pests. As far as the crowns are concerned, the most common crossbreeding was between the Pera sweet orange and the Murcott tangor (a natural hybrid of an orange with a tangerine).
The basis of São Paulo’s orange growing, where it accounts for 45% of the 200 million or so trees planted, the Pera sweet orange is easily attacked by citrus canker, CVC and leprosis, diseases that cannot develop fully in the tangerine. From this marriage of opposites, 311 different hybrids were born. In spite of being “children” of the same family, each hybrid of Pera sweet orange and Murcott tangor shows a genotype that is slightly different from the one found in its “brothers”.
In the case of crossbreeding between different kinds of rootstocks, the most common association was between the Poncirus trifoliata species, which belongs to a genus close to the citrus, and the Sunki tangerine. This marriage produced 281 genetically distinct hybrids. The objective is to achieve rootstocks that are more resistant to the Phytophthora fungus, which causes gummosis and the virus of citrus tristeza, a disease that, in the 1940’s, almost wiped out São Paulo’s orange groves, and today, in more attenuated versions, has become practically endemic in orange trees.
One point that needs to be made clear with regard to hybrids: it is not a question of transgenic plants, although the knowledge of modern genetics has been used in the orientation of some crossbreeds. Incidentally, one of the objectives of the Citros Genome Project is to produce a database of the genes of the orange, the tangerine and the Poncirus. The researchers generated 97,000 fragments of active genes of oranges and 12,700 of tangerines. When there is more information about the genome of citrus fruit, then indeed the creation of transgenic varieties, modified with genes from this same group of plants, should come onto the agenda.
1. Integration, Genetic Improvement, Functional and Comparative Genome of Citrus Fruits; Modality Millennium Institute (CNPq); Coordinator Marcos Machado – Sylvio Moreira Citriculture Center; Investment R$ 3,115,000.00