DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 1-2/1966 str. 113 <-- 113 --> PDF |
b) Multispecies hybrids This kind of hybrids may also exhibit the phenomenon of heterosis. In Placerville the hybrids (P. peuce x P. strobus) x P. monticola were produced which at the age of 11 years surpass in height the indigenous P. monticola. The same Institute also produced the hybrid (P. ponderosa x P. apacheca) x P montezumae, which grows even faster than the hybrid P. ponderosa x P. montezumae. We have obtained the trispecies hybrid P. silvestris x (P. densiflora x P. nigra) displaying a better growth rate than the Austrian pine A natural multispecies hybrid is P. nigra austriaca x (P. thunbergii x densi flora), which grows faster than its male parent. In the genus Castanea the trispecies hybrid C. mollissima x (C. crenata x C. dentata) is known which according to Wright , is very promising because of apparent resistance to Chestnut canker, from which the American chestnut suffers. As can be seen from the exmaples, this working method can also be very useful. Therefore when planning a long-term interspecific hybridization programme it is necessary to take these possibilities into consideration, particularly since we already prossess a large number of hybrids produced by the crossing of two different species. c) Application of irradiation We consider it necessary to mention still another possible method of producing heterotic plants. Geneticists engaged in the study of Drosophila have established that heterosis is often associated with deletions, inversions and translocations. In such a case one chromosome of the pair is completely normal, while the other originating from the other parent differs structurally from the first. For this reason, Osborn e (39) proposed that only one parent should be irradiated before crossing in order to be able in this way to induce heterosis. He believes that this method can be used with precision in long- lived forest trees. By irradiating the pollen and pollinating the nofiirradiated (normal) female gametophyte we should obtain heterozygous, heterotic plants. We should carry out selection among these plants in the course of cultivation. On the basis of the previously mentioned assumption, Rudolp h (4ß) made an experiment with Picea glauca. He tentatively concluded that irradiation of the pollen had a stimulating effect on seed set and viability. Pollen irradiated with 600 r also gave heavier seeds. The investigatios which we are conducting in this direction with Austrian and Scots pine show that plants of greater height and stem diameter can be obtained if the pollen is irradiated with gamma rays before pollination. The fallowing data are presented for the purpose of illustration: 3-year-old plants of Scots pine obtained through controlled pollination with non-irradiated pollen had an average height of 21.11 cm. and a stem thickness of 8.22 mm.; plants originating from pollen irradiated with 400 and 500 r had average heights of 23.8 and 26.78 cm., while their stems were 9.38 and 11.14 mm. in diameter respectively. HYBRIDIZATION BETWEEN RACES In his paper, in the title itself of the chapter on the interracial hybridization, Wrigh t (61) states rightly that this subject has been neglected. So far, there has been a very limited amount of work conducted on the crossing 111 |