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Breeding Objectives






Depending on the needs of a specifi c region of the world, the objectives of citrus breed- ing programmes may vary considerably. For example, some of the following objectives may have high priority in some regions and low or no priority in others.

 

 

Scion breeding

See Table 1.1 for the objectives of scion breeding.

 

 

Rootstock breeding

Nearly all commercial citrus in the world is grown as grafted trees, with the scion culti- var budded on a selected rootstock cultivar. A good scion and rootstock combination supports development of trees that bear large quantities of high quality fruit. Such a stoinic combination can maintain health and productivity for 50 years or more with modest management. However, many avail- able rootstocks are inadequate to meet the emerging needs and challenges. A large proportion of the problems faced by the citrus industry could be overcome by use of improved rootstocks (Wutscher and Hill, 1995; Bowman, 2000).


 

Benefi ts of the rootstock

EARLY BEARING. Budding a mature scion on to a rootstock plant takes advantage of the scion variety that has already passed juve- nility which can start production within 1–2 years of planting in the grove, whereas citrus seedlings typically require many years to begin bearing fruit when grown from seed (3–15 years, depending on the species). In addition, citrus seedlings often maintain undesirable juvenile characteris- tics, such as excessively large thorns, for many years after fruiting begins.

 

TRUE TO TYPE. Propagation by budding/graft- ing on to a rootstock ensures that the trees will produce fruit that is identical to the source of budwood and thus allow plant- ings of a uniform type.

 

APOMIXIS. Most citrus rootstocks are apomic- tic, which can produce uniform plants from seed at a low cost. Seed propagation costs less and produces more vigorous and uni- form nursery stock than by cuttings or tissue culture.

 

COMBINING TRAITS. Propagating citrus through budding on a rootstock is the benefi t that probably has the most relevance in terms of rootstock breeding. It allows the graft creation of a tree that combines the best genetic fruit characteristics above ground with the strongest genetic root traits (adaptations to soil type, tolerance to salinity, resistance to diseases and nematodes, etc.) below ground as two separate units. Creating such a combination


 

 

 
 

Table 1.1. Objectives of scion breeding.

Tree performance Fruit characteristics Postharvest of fruit

Yield Exterior appearance Handling for fresh market

Cold hardiness Size and shape Economic and cultural importance Adaptation to adverse climatic and Quality (ss/acid ratio) Processing quality (processed into frozen soil conditions Juice content, concentrate or single strength juice

fl avour/colour products) Adaptation to mechanical harvest Ease of peeling

Disease and pest resistance Seedlessness Storage life

 
 

Season of ripening Juice content and composition


 


of genetic traits in a single (self-rooted) geno- type would be considerably more diffi cult.

 

Problems created by using a rootstock

EXPERTISE. Budding/grafting for propagation requires a signifi cant expertise and adds cost when compared with seed propagation.

 

LACK OF APOMIXIS. A uniform nursery stand typically relies upon nucellar polyembry- ony (Frost and Soost, 1968) which results in the production of genetically uniform clonal seedlings. If a rootstock cannot be propagated uniformly by seed, clonal prop- agation must be accomplished by cuttings or tissue culture. The problem is further compounded when a rootstock strain pro- duces a mixture of nucellar and zygotic seedlings which will require a rouging pro- cedure (Khan and Roose, 1988).

 

GRAFT-TRANSMISSIBLE DISEASES. Vegetative propa- gation of the scion by budding can carry many virus and virus-like diseases and other pathogens. Hence, citrus has a long list of graft-transmissible pathogens that can debili- tate trees and must be carefully excluded from propagation material (Roistacher, 1991).

 

GRAFT INCOMPATIBILITY. An important problem associated with using a rootstock is poten- tial graft incompatibility between certain rootstocks and scions. Although generally not well understood, graft incompatibilities apparently arise when there is a confl ict between the physiology of the rootstock and scion that causes the tree to grow


weakly or die. A similar reaction can be induced by some virus diseases, such as incompatability which occurs when trees of sweet orange (Citrus sinensis) on sour orange (C. aurantium) are infected by iso- lates of citrus tristeza virus (Bowman and Garnsey, 2000).

 

ROOTSTOCK AFFECTS THE SCION. It must also be

kept in mind that rootstock does affect many important traits of the scion grafted on to it, including tree size, productivity and fruit quality. Hence, a rootstock may degrade the tendency of a scion to be highly productive or to yield good quality fruit. One example of this is the rough lemon (C. jambiri) rootstock, which produces a vigor- ous and highly productive tree, but also induces the scion to yield low quality, low sugar fruit.

 

 






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