Studies on the Adventitious Root Formation of Japanese Native <i>Rhododendron</i>

Some methods of cutting in a propagation house equipped with intermittent mist and the effect of auxin treatments on rooting were investigated from 1973 to 1977, using as the mature stock plants R. metternichii var. hondoense growing in the mountains of Gifu prefecture.<br>Stem cuttings were taken from 10 to 20 year old plants in the middle of July after the current shoot elongation had ceased. The period of cutting extended over four months.<br>1. As a propagation bed soil for cuttings, a medium consisting of 7/10 Kanuma- tsuchi and 3/10 peatmoss or akadama-tsuchi were more suitable for rooting than Kanuma-tsuchi alone. Both a medium consisting of 7/10 Kanuma-tsuchi and 3/10 coarse sand, and a common field soil were inferior, and a medium consisting of 7/10 Kanuma-tsuchi and 3/10 chopped sphagnum was the most inferior for cuttings because a lot of dead stem cuttings occured. The physical conditions of the bed soils, investigated at the end of the experiment, had little difference among them, but it was judged that the medium of Kanuma-tsuchi and peatmoss was higher in water holding capacity than the Kanuma-tsuchi alone.<br>2. Many more two year old branch cuttings rooted than current shoot cuttings, but current shoot cuttings rooted to the same degree in some cases; therefore, the influence of the stem cuttings′ age on rooting could not be made clear.<br>No influence on rooting of different leaf areas among 3, 5 or 7 leaves per stem cutting was found. Within the limits of five leaves per stem cutting, restriction of leaf area due to cutting off half of each blade had no effect on increased rooting.<br>3. In regard to the effect on rooting of auxin treatment at the base of stem cuttings, IBA had little or no effect, but NAAm was effective. Dipping stem cuttings into 35°C hot water or 2% ethanol as a pre-treatment was not injurious to their growth, and the former effectively increasing the rooting compared with non pretreated stem cuttings.<br>4. First most of the stem cuttings began the callus formation at their cut surface 10 to 30 days after the beginning of cutting, and then they began the root differentiation 30 to 60 days after. Water content in the leaves of stem cuttings reduced continuously with the beginning of cutting, but it became constant after 60 days. Dry matter content in the leaves of stem cuttings reduced temporarily when they formed a callus and after that recovered slowly, and such stem cuttings survived although they still did not root.<br>5. The highest percentage, of rooting, 65%, was obtained from only 2 year old branch cuttings from which the current shoot had been removed, and which were treated with NAAm and placed in bed soil consisting of Kanuma-tsuchi and peatmoss. This effectiveness may result from having removed the terminal bud, which contains the developing flower bud.<br>The better cutting results in this experiment were obtained by rooting a current shoot cutting into bed soil consisting of Kanuma-tsuchi and peatmoss, and a two year old branch cutting into bed soil consisting of Kanuma-tsuchi and akadama- tsuchi. The ratio of rooted cuttings in these cases was 53% and 48%, respectively.<br>Although these results may be unsatisfactory in view of practice, the callus formation was vigorous in the case of the cuttings which did not root. Therefore, an increase of rooting could be expected if physiological factors of stem cuttings in relation to rooting could be made clear, and a cutting method and its environmental condition could be improved.