GROWTH-PERFORMANCE OF HYBRID FAMILIES BY CROSSING SELFED LINES OF BETULA-PENDULA ROTH

Hybrid breeding is an effective approach in many agricultural crops. I n allogamous tree species severe inbreeding depression and long reprod uctive cycles generally prohibit its use. However, three generations o f selfing in silver birch (Betula pendula Roth) were obtained by forci ng trees to flowering under greenhouse conditions. Hybrids were produc ed by crossing first-, second- and third-generation selfed lines. The effects of different levels of parental inbreeding on the growth perfo rmance of hybrid families were observed in a 9-year-old field progeny test. Also, provenance crosses were carried out between selfed lines f rom different parts of Finland and several other European countries. O bservations of growth performance of the provenance hybrids were made in the same trial. The results indicated that the mean stem volumes we re significantly different between classes of parental inbreeding coef ficients (F-p) (P<0.0001), and were positively correlated with F-p (r= 0.9106, P<0.05). Within-family variation of the hybrid families decrea sed with an increase of F-p. The performance of the provenance crosses between parents at a relatively close distance did not depart signifi cantly from the standard controls. However, when the cross distance wa s extended far to the south, hybrids grew faster, indicating either hi gher heterozygosity or an extended growth period.