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548 C. HEIMBURGER 



species at all which may attack the ¥\ very badly and then you're sunk. 

 I'd rather say that the F]_ is a means to an end. That we have to have 

 the F2 or a backcross before we can do anything with it, and we have as 

 yet not discovered a method of producing F2's without first producing 

 Fi's. 



BINGHAM: What is the frequency of the precocity in P. peuce? 



HEIMBURGER: About 1/3. 



BORLAUG: Is this pretty much the same frequency in the interspecific 

 crosses? 



HEIMBURGER: Yes. So the gene must be quite widespread in P. peuae. 



CAMP AN A: How long have you been able to store pollen of white pine 

 and retain viability and under what conditions? 



HEIMBURGER: We store white pine pollen over silica gel in a dessicator 

 at 20 below zero F.. and we can store it for 2 years, but we don't like 

 to store it more than 1 year. 



GERHOLD: Dr. Heimburger, can you make any general observations about 

 adaptation and growth form of the various types of hybrids, including 

 F2's, backcrosses, and triple hybrids? 



HEIMBURGER: Well, I think that the cross P. peuae x P. griffithii 

 is much more promising than the cross P. peuce x P. strobus , because the 

 Fi's with P. strobus are rather miserable trees and they grow slowly, and 

 are infected by several insects, which P. peuce and P. strobus are not. 

 The P. peuce x P. griffithii approach is probably better in the long run; 

 Pinus peuce of course, grows farthest north in our area of all the exotic 

 species. We have the native white pine growing up to Lake Nipigon and 

 then P. peuce, Pinus monticola and P. griffithii are more tender. So we 

 have used P. peuce quite a bit more than any other because of that. We 

 have grown it up north and tested it and I would say that hybrids between 

 P. peuce and P. griffithii are quite promising with us. We have several 

 P. griffithii hybrids in parks in Toronto where people have obtained seeds 

 from Italy under the name of P. griffithii, and of course the hybrids grow 

 faster, and survive under hard conditions and they are now placed in parks 

 under a P. griffithii label. They are not P. griffithii; they are 

 hybrids. The backcrosses to P. strobus are quite good. They can easily 

 be crossed with P. strobus. Pinus peuce is not so easy, and of course, 

 P. parviflora is the most difficult. 



ZUFA: In F2 and following crosses, as well as in the backcrosses, 

 we have segregation back to the original species and forms and we sometimes 

 produce very heterogeneous populations. I wonder what your opinion is 

 about that and how do you intend to use such populations in forestry 

 practice? 



HEIMBURGER: I don't think that we should worry much about uniformity 

 because the first backcross of course will be under laboratory conditions, 

 so to speak. The second backcross is of interest to forestry, and by that 

 time the variations should be reduced to practical levels. In fact, it 

 would be very useful to have some variation to maintain expression of 

 dominance and we propose to screen them for blister rust resistance every 

 time and then let them grow under plantation conditions, and eliminate 

 all the poor ones. 



