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Introduction. All the world’s citrus industries will bene- fit from genetic improvements leading to the release of superior new rootstock and scion cultivars






All the world’s citrus industries will bene- fi t from genetic improvements leading to the release of superior new rootstock and scion cultivars. Improvements in pest and disease resistance, tolerance of various environmental stress factors, horticultural performance and productivity, and fruit quality characteristics should result in greater economic returns to growers, processors and shipper/packers by reduc- ing input unit costs and increasing product desirability and demand. Likewise, improvements in fruit quality, nutritive value and content of health-protective com- ponents should benefit consumers and increase consumption of this well recog- nized, nutrient-rich fruit. Until recent times, however, much of the process of genetic improvement of citrus was based on identification of chance mutations and seedlings displaying useful characteristics. Structured and targeted breeding pro- grammes were generally inefficient and ineffective because of a lack of genetic


knowledge of important traits, incomplete understanding of the signifi cance of taxo- nomic distinctions and relationships, and the absence of breeding tools that could be employed to achieve necessary and desired goals. However, in the past two decades, there has been a tremendous explosion in the understanding of plant genetics and genomes, as well as associated technologies to enable application of new fundamental knowledge to genetic improvement of crop plants in general, and specifi cally of citrus; this volume itself is a testament to that tech- nological revolution, specifically as it relates to citrus. As a consequence, a com- prehensive approach to citrus genetic improvement and cultivar development, utilizing the most appropriate methods and technologies to achieve given improvement goals, makes the greatest sense.

The authors of this chapter are mem- bers of a team dedicated to developing genetically superior new citrus cultivars, in a comprehensive fashion. The individual members bring diverse expertise to the team, from fundamental genomics and


 

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 9


genetics, from tissue culture-based systems of somatic hybridization and genetic trans- formation, from traditional breeding tech- niques frequently assisted by tissue culture methods, to screening newly created mate- rials for specifi c traits and characters in spe- cialized challenges, and ultimately proving the value of new selections to growers and for commercial release following extensive replicated fi eld trials. The basic elements of the University of Florida’s citrus genetic improvement and cultivar development programme are outlined in Fig. 2.1. All of the research efforts of our team, whether they are traditional breeding for multitrait improvements or genetic experiments to


clone genes that can precisely modify a spe- cifi c trait, are directed ultimately toward the central and unifying goal of releasing new and improved rootstock and scion cul- tivars for the industry. All forms of genetic modifi cation, listed in general terms in Fig. 2.1, yield new plants that represent a pool of genetic diversity upon which screening and selection are imposed to fi nd and to verify truly superior performing individu- als for release as new cultivars. Contributions from genomic research feed into the process as cloned genes that can be introduced to the pool via transformation; alternatively, genomic research also leads to the development of screening tools via


 

 

 

Fig. 2.1. Overview of the basic elements of the genetic improvement process for citrus.


 


marker-assisted selection (MAS), which enables much more effi cient selection of superior recombinants improved for multi- ple traits from conventional breeding efforts. Once superior candidate individu- als have been identifi ed; their value and performance are assessed and documented in many locations and conditions to iden- tify the most valued of the candidates for release.

The information and the fi gures con- tained within this chapter are extracted from a working document, titled ‘A Comprehensive Citrus Genetic Improve- ment Program’, which the team uses as a guide and a plan for the research efforts in which we are engaged; the document is reviewed frequently and updated periodi- cally as needed. This document has been provided to various stakeholders in the process, including university administra- tors, citrus growers, funding agencies, researchers and any other interested indi- viduals or groups, such that the compre- hensive nature of citrus genetic improvement as we view it can be under- stood and supported. It represents an effort to explain in lay terms some of the com- plexities of genetic science from the per- spective of outcome, and not necessarily from the perspective of the details of the science itself. As such, it enables adminis- trators and funding agencies to approach questions relating to the needs of the pro- gramme, as well as its potential to deliver new improved products, from a holistic point of view.

 

 


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