IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 585 



For completion of one breeding cycle 10 to 26 years 



(Selection of parents for the first 

 controlled sibbing, back crossing , or 

 mass-pedigree breeding) 



To provide first-cycle tested ortets 10 to 24 years 



(For multiplication for use in multi- 

 clonal hybrid varieties) 



SUMMARY 



The overall objective of mass breeding methods for multiple-trait 

 improvement of forest planting stock with emphasis on pest resistance 

 should be the creation of varieties with superior genetic fitness. This 

 will require both populational and individual buffering. 



Mass production of improved planting stock may be by any or all of 

 the following procedures: (1) Single- or multiple-cross hybrids; (2) 

 varietal blends; (3) multilineal varieties; (4) synthetic varieties; (5) 

 multiclonal varieties. 



The evidence from crop breeding indicates that the best F^ hybrids 

 are generally superior to synthetic varieties. Selfing and the production 

 of T± hybrids have been powerful tools for improvement of cross-fertilizing 

 crops; with the possibilities of diploidization of haploids and cyto- 

 plasmic male sterility, they can be equally valuable for improvement of 

 forest trees. 



Increased population buffering to enhance fitness of mass-produced, 

 rust-resistant hybrid or synthetic varieties of forest trees could be 

 obtained through seed blends. Mass production of "multilineal seed 

 blends" will depend on practical methods for the production of hybrid 

 lines. The development of such blends resistant to different races of 

 the rusts will not be possible until rust races have been identified; and 

 new races of the rusts would not be expected to develop until there are 

 extensive plantations of a rust-resistant variety. 



Clonal and seedling seed orchards are the base populations — the Syn-0 

 generations --for the production of synthetic varieties that are propagated 

 by seed. The open-pollinated progenies of clonal orchards after roguing 

 that has been based on progeny tests will be Syn-1 synthetics; the open- 

 pollinated progenies for seedling seed orchards after phenotypic roguing 

 will be "mass-selection" synthetics. Seed-increase to produce Syn-2 

 foundation seed and Syn-3 certified or registered seed (as in agricultural 

 crops) is neither practical nor anticipated for forest trees; seed orchards 

 are generally large enough to meet the forestation requirements of the 

 organizations that establish them. Genetically improved synthetic 

 varieties will require parental populations with high general combining 

 ability, not only for special traits, but also for a high degree of 

 fitness . 



Multiclonal hybrid varieties would be mixtures of clones (intra- 

 or interspecific hybrids, or both) selected for a high degree of vegeta- 

 tive fitness and for special traits, such as growth rate, timber form, 

 and resistance to pests. The genetic gain will depend upon the average 



