Scion Protocol

Various sources of genetically variant plant materials are listed in Fig. 2.2. These include sources that have been used for many of the advances over centuries of citrus scion improvement: seed and bud- wood introductions from elsewhere, or nat- urally occurring mutations that may be found. Other types of mutations are also exploited, such as those that arise from tissue culture cycles (somaclones) or from

induced mutations following irradiation. Given the taxonomic nature of many of the commercial citrus cultivar groups (sweet oranges, grapefruit, lemons and several types of mandarins) and their naturally occurring diversifi cation through somatic mutations, these groups are not amenable to breeding strategies based on sexual hybridization. Therefore, mutational approaches to genetic improvement have been and will continue to be critical, along with the great potential now in hand by virtue of the ability to transform such culti- vars with targeted genes. Also included in the list of sources of genetic variation are plants that arise from hybridization by sexual or somatic processes. Although the goal is to create new cultivars, the process likewise continues to identify new recom- binants to be used as breeding parents in cases where breeding remains a viable improvement strategy. The protocol is robust; thus, plant material from any source or developed by any technique can be entered into the system at any point.

Seedlings, irradiated budlines or other scion candidate materials are propagated on to rootstocks or grown on their own roots, planted in a fruiting-out orchard, and potentially desirable or valuable individu- als are selected according to their fruit and tree characteristics. Evaluations for specifi c characters that can be accomplished prior to fi rst fi eld planting (e.g. simplifi ed disease resistance selection, the presence and/or performance of transgenes, etc.) take place when possible. Upon fi rst fruiting, evalua- tions are made of both tree and fruit charac- teristics, with the objective of identifying those individuals that may potentially prove valuable as new cultivars. This objec- tive is accomplished by a process of elimi- nation of individuals with undesirable fruit characteristics, and by selection of individ- uals exhibiting positive characteristics (appropriate fruit size, colour, fl avour, pro- ductivity, and so on). These observations are made for three years at least, to assess stability of trait expression over time and to avoid excluding potential candidates that may appear undesirable during their fi rst

Fig. 2.2. Protocol for citrus scion improvement.

fruiting. The specifi c criteria for selection are determined by the team member pro- ducing the given materials, based on the type of fruit and the specifi c objectives of the genetic manipulation performed. The outcome of this phase of the protocol is a collection of candidates for advanced fi eld evaluations, as well as the identifi cation of individuals that can be used in further breeding cycles based on their expression of desirable combinations of genes and alle- les.

Individuals selected from primary screening are propagated on to one or sev- eral rootstocks and planted in replicated fi eld trials in multiple locations represent- ing the environmental diversity in Florida. The objectives of this stage of the protocol are to identify truly superior individual selections and to acquire adequate informa- tion on their performance, in comparison with standard cultivars, to warrant further development leading to possible release of new cultivars. These objectives are accom-

plished by evaluating horticultural per- formance, susceptibility/resistance to pests and diseases, regional environmental effects on performance and quality, root- stock interactions and other relevant fac- tors. Also important at this stage is the identifi cation of special problems that may be encountered by growers attempting to produce fruit from these new cultivars. At times, opportunistic plantings may be made, for example, to test for resistance to a pathogen that may be widespread in a par- ticular growing area; such plantings are not replicated fi eld trials, but none the less they can provide valuable insight and informa- tion regarding the performance and poten- tial of certain primary selections.

As data are collected from replicated fi eld trials, fi nancial analysis is used as a tool to determine the potential value of new processing cultivars compared with exist- ing ones, based on yields and qualities of the fruit of the new cultivars. The perform- ance and potential value of new scion culti- vars intended for the fresh market can be better determined by test marketing. This phase of the protocol is applied only to the most promising, elite selections identifi ed through previous stages of the protocol. Testing may be of the potential of processed products, as determined by sensory panel analysis, biochemical profi les as related to fl avour and quality, and other approaches. Cultivars intended for the fresh fruit market can be test-marketed by cooperating agen- cies and organizations, on a small but com- mercial scale; fruit performance in important aspects of the market channels is carefully evaluated, including handling ability and consumer/retailer preferences and responses. Poor performance or results from economic analyses or from test mar- keting would probably preclude release of a given selection, in most cases.

As the latter stages of the protocol are entered, sources of budwood of the elite selections are established, to have suffi cient propagation materials available for rapid multiplication and commercialization at the time of release. These selections must be certifi ed through the Citrus Budwood

Registration Bureau, Division of Plant Industry (DPI) of the Florida Department of Agriculture and Consumer Services (FDACS), to ensure the distribution of dis- ease-free budwood to nurseries for propaga- tion as required by state law. The programme supports an insect-proof repos- itory facility in which pathogen-free bud- wood sources of advanced selections are held and increased as needed, for fi eld trials or to provide budwood to the nursery industry if necessary. Throughout the entire process, measures are taken to pro- vide appropriate protection for new culti- vars and selections; these measures include the use of Material Transfer Agreements when materials are tested in the orchards of commercial growers cooperating in the evaluations, as well as patent protection prior to offi cial release. When the protocol has been completed and the potential value of new scion selections has been demon- strated, then the cultivar(s) will be released upon acquisition of a plant patent and licensing of nurseries to produce trees for the industry and/or growers to produce the crop. Two types of releases are possible: fi rst, the release of cultivars to the commer- cial industry; and, second, releases for selections that would be of interest to the home gardening clientele. The latter may be released following a less stringent approach to the protocol, compared with those to be released commercially.