IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 583 



with selected high-GCA candidates from each preceding stage, would each 

 be the Syn-0 generation for new Syn-1 synthetics. 



Progeny test methods, using controlled pollinations with individual 

 male testers or pollen mixtures, are practical and have been developed 

 for clonal seed orchards. Seedling seed orchards are also half-sib progeny 

 tests and should be designed as such. They would be rogued to leave the 

 best individuals of the best progenies. The planting stock produced after 

 the first roguing would be a "mass-selection" synthetic; theoretically, 

 with a broader genetic base and wider cryptic variability than the Syn-1 

 from a clonal orchard composed of a relatively small number of clones. 



The evidence from crop breeding on the effect of the number of 

 individual genotypes in the Syn-0 generation is based largely on the 

 performance of later than Syn-1 generation synthetics because of the need 

 for seed-increase. The effect of the number of Syn-0 genotypes on the 

 Syn-1 generation of wild forest tree species is an open question. Syn-1 

 synthetics may be different from year to year; each year will provide 

 possibilities for variation in the pollination pattern and the individual 

 seed productivity of individual trees. This will be reflected in a 

 variation of the genotypes present in the synthetic variety in different 

 years. Therefore it will be advisable to label the production of seed 

 orchards by "vintage" years . 



MULTI CLONAL HYBRID VARIETIES 



During the past 30 years, I have emphasized the importance of the 

 clone for forest tree improvement, the need to develop practical methods 

 for economical asexual propagation and, in recent years, my conviction 

 that maximum genetic improvement will be achieved and maintained through 

 multiclonal hybrid varieties (Schreiner, 1939 , 1958, 1960, 1963a, 1963b, 

 1966a, 1966b, 1967) . 



A multiclonal hybrid variety would be a mixture of many hybrid clones 

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

 vegetative fitness and for important special traits such as resistance to 

 pests. In the case of hybrid poplars, the availability of exceptionally 

 superior clones has resulted in very extensive monoclonal cultures. The 

 hazards of monoclonal cultures of forest trees have been pointed out , 

 independently, by Hartley (1939) and by Schreiner (1939) , and in recent 

 years by a few other writers. 



The use of multiclonal varieties requires economically feasible 

 methods for asexual propagation of superior genotypes; at present, very 

 few forest species can be vegetatively propagated for commercial foresta- 

 tion. Since basic research on vegetative propagation of forest trees is 

 now well underway and increasing, I believe practical methods for clonal 

 propagation of important but diffi cult-to-root forest species will become 

 available within the next decade. Winton (1968) has produced complete 

 plantlets in tissue culture from callus of a triploid Populus tremuloides . 

 Wolter (1968) has controlled root and shoot formation in callus cultures 

 of Populus tvemuloides with auxin (NAA) and cytokinin (BAP) , respectively. 

 Brown and Lawrence (1968) are currently growing callus cultures of Pinus 

 palustris s P. taeda, P. elliotti, and of Thuja 3 Lavix, and Picea species 

 to investigate the factors involved in propagating diffi cult-to-root 

 species. 



