DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 1-2/1966 str. 114     <-- 114 -->        PDF

of races within one species, although in many cases good results can be
expected from such work. It is well known that in agricultural plants it is
possible by crossing various races to obtain heterotic hybrids. The same can
also be achieved in forest tree species. Naturally, a necessary prerequisite to
this is the determination of the races, varieties and subspecies occurring in
a species. The more clearly separate the species and taxonomic units, the
greater is the probability of obtaining by crossing them a progeny which v/ill
have a good growth rate and even exhibit hybrid vigour. Thus, Johnsso n


(27) in Sweden, by crossing aspen of a southern provenance with that of a
northern provenance, obtained progenies which at the age of ten years were
more vigorous than either parent. This possibility is also confirmed by the
results of van V 1 o t e n (57) and Schönbach (51) after crossing various
provenances of aspen.
An instrucitve example of the value of the crossing of lower taxonomic-
units within a species exists also in this country (Istria). In a plantation of
Pinus nigra we observed that great differences exist in the morphological
characteristics of individual plants. Some of their characteristics can be found
in Corsican pine, others in Austrian pine, while a number of them are intermediate.
There are no written data on the history of this culture but it is
most probable that the seeds from which these plants were raised were
collected from a mixed plantation of Austrian and Corsican pines. From visual
observation it is apparent that the intermediate plants — which we assume
to be hybrids of Austrian and Corsican pines — exhibit greater stem heights
and diameters b.h. than Austrian pine.


L a n g n e r (31) explained the heterosis in interracial crossing of Norway
spruce. The race from the north grows in its native environment under long-
day conditions. With respect to the daylength required for its growth, it is
homozygous and possesses a genetic constitution of the LLkk-type. The
southern race grows under shorter-day conditions, and in this character its
genetic constitution is of the HKK-type. If these two races are crossed, they
give rise to progenies of the highest possible LIKk-heterozygosity. Such an
explanation is acceptable, although we krow that growth is conditioned by
several factors, and thus its interpretation is much more complex. We are
of the opinion that for the time being it is most important to pay attention
to the heterosis phenomenon, and as far as possible to guide hybridization
in this direction.


Interracial hybridization can have its value also if we do not obtain a
heterotic progeny. By crossing two races and the combination of their
characters it is possible to obtain progenies possessing the desired characteristics.
Norway spruce in Sweden is such a case; the cross between the
Swedish and German races grows better than the native race, which is just
as cold-resistant as the indigenous Swedish race.


DIFFICULTIES IN INTERSPECIES HYBRIDIZATION AND METHODS
OF OVERCOMING THEM


We often meet with difficulties in hybridization between species. A great
number of combinations in crossing give completely negative results. We
consider it necessary to say something about this problem. One should search
for the causes of negative results in the relationship of the species crossed,