542 C. HEIMBURGER 



The first crosses were largely failures, as the few seedlings obtained 

 perished in the seed beds and succumbed to blister rust. Starting from 

 1956, reasonably satisfactory results began to accumulate. Tables 1 to 3 

 list all the crosses made between 1956 and 1965, with the results obtained. 

 These comprise 20 combinations between species of the subsection Strobi 

 of Critchfield and Little (1966), seven of the subsection Cembrae and 12 

 between Stvobi and Cembrae. There are 21 new crosses, 3 successful with 

 Strobi* one with Cembrae and two between Strobi and Cembrae. In addition, 

 there are three F2's and 14 backcrosses, all of Strobi, as well as 14 

 triple and multiple crosses, ten of Strobi and four between Strobi and 

 Cembrae, the latter unsuccessful. 



Crosses of particular interest to the white pine breeding project 

 are those between P. strobus and P. griffithii, P. montioola Dougl. 

 P. parviflora and P. peuce. Triple and multiple crosses between these 

 species were also largely successful. The other crosses were mostly 

 exploratory, to gain more insight into the crossability patterns within 

 and between Strobi and Cembrae. In general, the results confirm the 

 information assembled by Wright (1959) and Little and Righter (1965) . 



The crosses P. armandii Franch. x P. lambertiana Dougl. and P. 

 koraiensis Sieb. £, Zucc. x P. lambertiana, involving a pine species of 

 great economic importance, were successful without resorting to embryo 

 culture which was necessary in the respective reciprocal crosses (Stone 

 and Duffield, 1950) . In nearly all interspecific crosses the number of 

 full seeds per cone and percentage of full seeds obtained were lower than 

 in the intraspecif ic crosses P. strobus x P. strobus. This applies also 

 to the F2 ' s and backcrosses and to the triple and multiple crosses. Thus, 

 breeding of white pines at the interspecific level must take reduced seed 

 yield into account, at least in producing the Fi and F2 generation hybrids. 



The information obtained thus far indicates the presence of a 

 relatively closely related group of five white pine species: P. griffithii, 

 P. montioola, P. parviflora, P. peuce and P. strobus, at least three of 

 which (P. griffithii, P. montioola, P. strobus) are of major economic 

 importance. They are easily crossable with each other, with some loss in 

 seed production following interspecific hybridization, and in fertility 

 of the resulting Fj's. 



Previously, it was found (Heimburger, 1961) that P. peuce carried a 

 dominant gene or genes for precocious flowering which was clearly expressed 

 at the interspecific level . This has been confirmed in the present 

 crosses. All crosses of P. peuce with species of the closely related 

 group mentioned above have yielded some seedlings with precocious 

 flowering. 



Three of the five species of the group are known to be more resis- 

 tant to blister rust than P. strobus and P. montioola. This resistance 

 is inherited in at least some hybrids with the more susceptible species 

 and is presumed also to depend on the breeding materials used. Compared 

 with P. strobus, no exotic species have proved to be as suitable for 

 forestry purposes in climatic and ecological adaptation and resistance 

 to certain native pests. This probably also applies to the other four 

 species in their respective homelands. 



Because of these findings and because of the decreased fertility of 

 the interspecific F^'s, the most promising approach in long-range resis- 

 tance breeding is the introduction of additional resistance genes from 



