DISCUSSION AND CONCLUSIONS 



The feasibility and efficiency of an inbreeding program depend upon five conditions: 

 Survival of inbred plants to fruiting age 



Fruiting within a reasonable time, and by a reasonable proportion 

 of the plants 



Success of the inbred pollinations, or attainment of useful levels 

 of filled seed production 



Germination, survival, and fruiting of successive generations of 

 inbred seed and seedlings 



Restoration of reproductive vigor (seed bearing) when inbreds are 

 finally outcrossed 



The record on the first four of these conditions is reasonably complete for first- 

 and second- generation inbreds of three of the species covered in the literature review 

 (Norway spruce, Douglas-fir, and Japanese black pine) , and it is here extended to western 

 white pine. The record of success and levels of seed production attained in second- 

 generation crossing of these four conifers is reviewed in table 5. 



Where the number of second-generation crosses attempted in these four conifers has 

 been adequate, generally there is fair crossing success. From 45 to 100 percent of the 

 crosses produced mature cones, and 25 to 100 percent produced filled seeds. 



Filled seed yields per cone have been low in S2, S^ X full-sib, and S^ backcrosses; 

 fair in S^ single crosses. Data on germination of second-generation inbreds are scanty, 

 and cover only three of the four species of table 5--Pinus montiaola only for seed 

 from Sx backcrosses, and Piaea abies only for seed from S2 crosses. Nevertheless, in 

 six tests (41 to 1,821 seed per test), the second-generation inbred seed has germinated 

 well--in the 40 to 80 percent range. For Pinus montiaola, Si seed germinated at the 

 50 percent level (Bingham and Squillace 1955) and there has been no further dropoff in 

 germination of S^ backcross seed. This confirms Andersson's (1965) observations that 

 reduction in germination occurred principally in tlie S^ seed. 



Similarly, survival of second-generation inbreds of Douglas-fir and western white 

 pine appears to be good. Orr-Ewing (1965) showed that first-year survival in those 

 second- generation inbred seed lots having more than 10 seed ranged from 30 to 90 percent, 

 and averaged 65 percent. Between 45 and 80 percent (average 57 percent) of the seed- 

 lings in five western white pine S^ backcross seed lots having more than 10 seed 

 survived 3 years in the nursery. Many of them are now 7 to 8 years old in the field. 



Righter (1962), and of course many maize breeders before him, have recognized the 

 "reproductive weakness" of inbred maternal parents. He states that "seed yield per 



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