Outlook

Tracking the ancestry of citrus is a complex process because of the great diversity and the distant centres of origin. Indeed, the evolution from wild citrus of Asia to modern day cultivars took hundreds of

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 1

thousands of years. Now grown in more than 100 countries in tropical, subtropical and Mediterranean climates, citrus (includ- ing oranges, grapefruit, tangerines and man- darins, and lemons and limes) is the leading fruit crop grown in the world. In 2004, world production of citrus was 108, 535, 000 Mt (www.fao.org) which is dominated by oranges (Fig. 1.1). Brazil and the USA (Florida and California) were lead- ing producers of sweet oranges. The USA is the primary producer of grapefruit. China, Spain and Japan produce 65% of the tan- gerines grown in the world. Lemons are produced primarily in Argentina, Spain and the USA, while Mexico is the largest producer of small fruited limes. Lime is also a traditional crop in South Asia and the Middle East. While consumers are gener- ally familiar with the edible citrus types, there is an equally large component of non- edible citrus used as rootstocks for success- ful production of edible citrus. Additionally, many citrus species have industrial signifi cance as a raw material for cosmetic and pharmaceutical products.

In general, the citrus production areas are located within 35° north and south of the equator. The main citrus regions, how-

ever, are in the subtropics, which are more than 20° north or south of the equator. As with most agricultural crops, many factors are known to limit the production and pro- cessing of citrus. Most are dependent on problems related to scion and rootstock deficiencies. Major constraints to citrus production involve management ineffi cien- cies, susceptibility to pests and diseases, and environmental challenges. These lead to increasing production costs, declining labour supply in many parts of the world and urban encroachment, especially on the most productive farm lands. Thus, new and improved scion and rootstock cultivars aimed at controlling these production and marketing constraints have been the pri- mary aim of citrus breeding efforts.

The development of new and improved citrus cultivars by conventional methods is a slow and costly process. It may take as long as 20–35 years or longer to release a new cultivar from the time of making the cross. Kinnow mandarin was bred and released at the University of California- Citrus Research Center, Riverside (Frost, 1935). The parental cross was made in 1915 and offi cial release took 20 years. However, it took another period of more than 30 years

Fig. 1.1. Proportion of citrus fruits produced in the world.

before Kinnow became a successfully grown commercial cultivar in the Punjab region of Pakistan and India.

Several serious obstacles exist that hamper citrus hybridization. For example:

(i) citrus is highly heterozygous; (ii) its unique reproductive biology such as apomixis and embryony; (iii) pollen and ovule sterility causing incompatibility; (iv) a long juvenile period taking as long as 5–10 years to express fi rst fl owering; and (v) adventitious embryos in the nucellar of developing ovules limit hybrid production. In developing seeds of polyembryonic cul- tivars, nucellar embryos compete with zygotic embryos. In addition to the long generation cycle, large seedling populations are needed (depending on species) that require extensive fi eld space and labour.

The fi rst formal citrus breeding pro- gramme was started by USDA in Florida in 1893 (Cooper et al., 1962) which is still expanding. The University of California established the Citrus Research Center, Riverside in 1907. The University of Florida initiated its citrus breeding pro- gramme in 1924 which is now one of the largest breeding programmes, centred at the Citrus Research and Education Center, Lake Alfred (CREC). Today, there are numerous citrus breeding programmes spread in all major citrus-producing countries.

Most of the present day scion and root- stock cultivars of citrus are the progeny of chance seedlings or a mutant branch of a tree, called ‘budsports’. The commercially successful cultivars now grown have resulted from the selection, propagation and advanced testing of thousands of such superior chance seedlings. This process of screening of potentially superior scion and rootstock candidates under commercial conditions involves a critical evaluation of tree performance and fruit quality traits. The above-mentioned breeding pro- grammes played a major role in shaping today’s citrus industry by providing infor- mation on the use of numerous scion and rootstock cultivars.

Breeders identify the most advanced candidates selected from initial screens and

usually propagate them on certain root- stocks, establish them in replicated blocks in different regions and environmental con- ditions, and compare them with known cul- tivars. After such rigorous evaluation of horticultural performance, economic analy- sis and test marketing for fresh market and processing use, superior survivors may fi nally result in the introduction of a named variety.

In some citrus breeding programmes, new selections are patented for economic and scientifi c protection. After receiving a patent, the new cultivar may be licensed to nurseries to multiply the trees for commer- cial distribution. Certifi cation programmes may be established to ensure that disease- and pest-free budwood is provided to com- mercial nurseries.