DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 1-2/1966 str. 112     <-- 112 -->        PDF

In the interspecific hybridization in elms, Albenskij also obtained progenies
exhibiting a better increment than the female parent. These crosses were
Ulmus pinnatoramosa x U. laevis.


OTHER POSSIBILITIES FOR OBTAINING HETEROSIS IN HYBRIDS


a) Inbreeding


The inbreeding method, well-known in the improvement of allogamous
agricultural plants, has the objective of obtaining heterosis in the final
crossing of two inbred lines. Since most forest tree species are allogamous,
this method can also be used in forest trees. This is feasible because it is an
established fact that it suffices to produce the Ii to IK generations. However,
there is a difficulty in cases of allogamous forest tree species which are
totally self-sterile. There is still insufficient information on this problem as
indicated by the fact that there are conflicting reports in the literature on
some species. Also, self-fertilization varies from tree to tree (43, 44). Total
self-sterility has been estalished in Castanea mollissima, Liriodendron tulipifera,
and in several species of the genus Eucalyptus. Almost complete self-
sterility has been established in Alnus glutinosa, Larix decidua, Larix leptolepis,
Picea sitchensis, Pinus strobus, Pinus silvestris, Pinus nigra, and others.
Species such as Pinus monticola and Picea abies have a varying degree of
sterility and inbreeding depression. The self-fertile species include Alnus
incana, and also several species of the genus Eucalyptus. The inbreeding
depression in these species varies from very little to almost 50%. It is also
important to mention that the seed obtained by self-fertilization is of poor
germinability, its progenies show a poor growth rate, and single individuals
often display an abnormal habit. Previous improvement work carried out by
this method has generally been in the first stage, i.e. the production of Ii and


I» generations. There are several examples which show the general responses
to inbreeding. L a n g 1 e t (29) showed that in Norway spruce inbreeding
decreased the percentage of germination. Plants in the first year of growth
already have a depression which in later years slightly exceeds 50%. This
inbreeding depression was also ascertained in other species, such as Pinus
silvestris, Pinus Mugo and Larix decidua. Jovanović and Tucović (28)
obtained from a monoecious tree of Black poplar an Ii-generation which did
not show inbreeding depression in the first two years of life. S y r a c h
L a r s e n and his co-workers have already obtained the la-generation in
European larch. A further stage of this procedure is the mutual crossing of
inbred lines with the aim of obtaining heterotic progenies.


If we carry out improvement by simultaneous inbreeding of two different
races or provenances, the possibility of obtaining a degree of heterosis is in
our opinion greater than if we perform this work within one race or provenance.
L a n g n e r (31) also recommends the use of different provenances
for improvement by inbreeding. Thus, for instance, N i 1 s s o n (38) obtained
an intensive heterotic effect in crossing one Swedish provenance cf Norway
spruce with one continental provenance. The author explains that the
heterosis phenomenon occurred because naturally-existing inbreeding depression
within each provenance, resulting in heterosis in the progenies.