Growth performance of hybrid families by crossing selfed lines of Betula pendula Roth.

Hybrid breeding is an effective approach in many agricultural crops. In allogamous tree species severe inbreeding depression and long reproductive cycles generally prohibit its use. However, three generations of selfing in silver birch (Betula pendula Roth) were obtained by forcing trees to flowering under greenhouse conditions. Hybrids were produced by crossing first-, second and third-generation selfed lines. The effects of different levels of parental inbreeding on the growth performance of hybrid families were observed in a 9-year-old field progeny test. Also, provenance crosses were carried out between selfed lines from different parts of Finland and several other European countries. Observations of growth performance of the provenance hybrids were made in the same trial. The results indicated that the mean stem volumes were significantly different between classes of parental in breeding coefficients (FP) (P<0.0001), and were positively correlated with FP (r=0.9106, P<0.05). Within-family variation of the hybrid families decreased with an increase of FP. The performance of the provenance crosses between parents at a relatively close distance did not depart significantly from the standard controls. However, when the cross distance was extended far to the south, hybrids grew faster, indicating either higher heterozygosity or an extended growth period.