546 C. HEIMBURGER 



the exotic species (donor species) to the native species (receptor 

 species) by means of backcrossing with the native species as the 

 recurrent parent. This will lead to the production of synthetic 

 varieties. A successful incorporation of exotic resistance genes into 

 the genome of a native species should result in strains with fully 

 recovered fertility and adaptation, permitting free gene exchange with 

 the most advanced strains of the native species at any point of an intra- 

 species resistance breeding program. 



The time factor is the most important bottleneck in such a breeding 

 program, consisting of artificial, guided introgression from one or 

 several exotic species into the native species. It takes about 25 years 

 for seedlings of P. strobus to begin flowering on a sufficient scale for 

 breeding purposes. It will probably take at least two backcrosses, each 

 requiring 25 years, to successfully incorporate exotic resistance genes 

 and thus to broaden the gene pool of the native species. This means a 

 period of about 75 years, and any means to shorten the breeding cycle of 

 such materials will be of major importance. 



To date there are two known methods of shortening the breeding cycle 

 of forest trees. One is the application of growth retardants and flower- 

 promoting hormones, and the other is artificial aging by shortening the 

 juvenile phase in a growth chamber (Dormling, Gustafsson, and von Wettstein, 

 1968). A third method, herewith proposed, is the introduction of domi- 

 nant genes promoting flowering, in this case genes from the precocious 

 P. peuce mentioned above. It should be possible to incorporate precocious 

 flowering from P. peuce into all the other related white pine species of 

 economic importance by means of appropriate backcrosses. Since preco- 

 cious P. peuce and some of its hybrids begin to produce female flowers at 

 age 5 and are flowering abundantly at age 7, this incorporation should 

 require at most 20 years. It is quite possible that flowering of 

 precocious materials can be further hastened by appropriate manipulation 

 of the environment. 



These precocious types are not suitable for direct use in forestry 

 because abundant flowering is usually achieved at the cost of wood produc- 

 tion and longevity, resulting in small, short-lived trees. However, the 

 precocious types could be used as immediate receptors of new resistance 

 genes to be introduced from any of the other related species. Such a 

 precocious receptor type could then be used to absorb the low fertility 

 and poor adaptation following interspecific hybridization with promising 

 new breeding materials from any exotic donor species. In three genera- 

 tions, i.e., in about 21 years, this program should produce strains 

 carrying the new genes in a form freely exchangeable with the most advanced 

 materials obtained at any point of the intraspecific breeding. 



Research with precocious P. peuce should be initiated to ascertain 

 how many dominant genes for precocious flowering there are available and 

 in what manner they can best be used for the purposes intended. The end 

 product should be a strain carrying the relevant dominant genes for 

 precocious flowering in a heterozygous condition, so they can easily be 

 eliminated in the final backcross to the receptor species. 



In grafts of P. strobus there is usually a gap of about 15 years 

 between female and male flowering (Wright, 1964). This has been a most 

 serious handicap in our white pine breeding work. Only recently Chiba 

 (1965) has found a method of inducing male flower formation in grafts of 

 P. strobus of flowering age. The method consists of pinching off all 



