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NFM, Volume 39: 47–58.
Original scientific paper

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Abstract

The cloning of silver fir trees by rooting of hardwood cuttings is allegedly feasible, even at older age. However, very scarse literature on this topic does not define a reliable method or technology that could be used for effective cloning of the species. By selecting plus trees in one base population, a breeding process of silver fir as a Christmas tree was initiated in Croatia. The next stage of the process is the optimization of the cloning procedure of selected plus trees for the mass production of their reproductive material (seedlings). It is assumed that in this way qualitative reproductive material exhibiting stability of targeted phenotypic traits will be produced.
The main aim of the research was to determine the success of various hormonal treatments on rotting of hardwood cuttings of older silver fir trees (20+ years). Two experiments were carried out. In the first experiment, 270 cuttings were cut in early spring from each of the three crown levels (upper, middle and lower third of the crown), on a sample of 10 trees. The cuttings from each level were treated with 24 different treatments (dipped in IBA and NAA hormone solutions of 250, 500, 2500 and 5000 ppm for 10 s, 1 h and 2 h). In addition to these treatments, 10 cuttings from each crown level were treated with a commercial hormone product named Clonex (a gel). Additional 10 cuttings were treated with distilled water for 2 h and the remaining 10 were not treated. The last two groups of cuttings served as a control of the success of the analyzed hormonal treatments. Another study was conducted on 80 cuttings cut in late fall from the lower third of the crown of 18 selected plus trees. Twenty cuttings were treated for 10 s with: (i) 2500 ppm IBA solution, (ii) 10000 ppm IBA solution, and (iii) 2500 ppm IBA + 500 ppm NAA solution. The remaining 20 cuttings served as a control. Afterwards, the cuttings were placed in containers filled with peat, sand and perlite substrate. The rooting success, the number of roots per cutting, and the length of the longest developed root were determined later in October.
The percentage of rooted cuttings was very low, regardless of the collection time of cuttings, their position in the crown or hormonal treatment (0–20 %). It was generally concluded that the cloning of silver fir trees by rooting of hardwood cuttings had a very low success. Applied experimental methods have not been shown to be effective for mass cloning of trees at the age of 20 and over. Hormonal treatment of cutting base increased the efficiency of their rooting and the quality of developed roots in relation to untreated cuttings although the obtained results were not convincing enough to allow practical recommendations. The highest success (20 % of rooted cuttings) was achieved by dipping cuttings taken in early spring from the lower third of the crown of trees in IBA hormone solution of 2500 ppm for 10 s. Generally, cuttings taken from trees at that age were not rooted without hormone treatment. However, specific genotypes whose cuttings have been rooted even without a hormonal treatment have been identified, although performing a very low percentage of rooting. Great intergenotypic variation was established, both in the ability to root untreated cuttings as well as in responses to hormonal treatment of cuttings.

Keywords: Christmas trees, breeding, plus trees, cloning, genotypes, IBA, NAA