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- Author or Editor: Lee Choong Il x
The level of endogenous root-promoting and inhibiting substances in 3 clones of rhododendron were compared at seasonal intervals in order to study the clonal and seasonal variation in rooting response of cuttings. The highest levels of 4 rooting cofactors in any season were found in both stem and leaf tissue of Rhododendron cv. ‘Cunningham’s White’ followed by ‘English Roseum’. The clone ‘Dr. H. C. Dresselhuys’ contained the least amount of the rooting cofactors. An inhibitor was often found in all clones, but it appeared less responsible for clonal differences in rooting response than variation in levels of the rooting cofactors. Rooting cofactor levels contained in the stem tissue of ‘Cunningham’s White’ were not less than those in the leaf tissue. In contrast, cofactor levels present in the stem tissue of ‘English Roseum’ and ‘Dr. H. C. Dresselhuys’ were less than those in the leaf tissue. The promoting activity of rooting cofactors in all tissues of the clones increased in September and decreased again in November to the level of July extract. The inhibitor found in the July extracts disappeared in September and reappeared in November.
Rooting of cuttings of ‘Dr. H. C. Dresselhuys’ was significantly improved by grafting a leaf and bud scion of ‘Cunningham’s White’. On the other hand, scions of ‘Dr. H. C. Dresselhuys’ resulted in decreased rooting of cuttings of ‘Cunningham’s White‘. Rooting capacity of ‘English Roseum’ was less affected by a leaf and bud scion of other clones of Rhododendron.
When Euonymus alatus Sieb. ‘Compactus’ plants were grown under intermittent water mist during September, leaf senescence and onset of dormancy were delayed and natural root-inducing phenolic and flavonoid compounds accumulated in the leaf tissues. Consequently, stem cuttings taken from Euonymus grown under intermittent water mist rooted easily, whereas cuttings from non-misted Euonymus rooted with greater difficulty. In addition, cuttings from misted stock plants rooted better in October than did cuttings from non-misted plants in early summer. When rutin, a flavonol similar to those which accumulated in misted leaves, was supplied exogenously with IBA to stem cuttings taken from non-misted Euonymus, the cuttings rooted as well as cuttings from misted plants. The results provide an additional explanation for the great success of modern mist propagation techniques and emphasize that many substances in addition to auxins and rooting cofactors may play a significant role in root initiation.
When Euonymus alatus ‘Compactus’ was grown under intermittent water mist for 2-4 months in autumn, the development of red foliage color was inhibited and dormancy and leaf abscission were delayed even during 4 weeks of cold temp (5°C) after the mist. Leaf tissues of misted Euonymus had a lower content of anthocyanins, total sugars, soluble N, and K and a higher content of starch, protein N, phenylalanine ammonia lyase, flavonols, flavans, leucoanthocyanins, and total phenolic substances than did non-misted plants. Substances involved in anthocyanin synthesis such as sugars, K, flavonoid compounds, and growth inhibitors were leached from the Euonymus by the mist. When the leached substances were added back to leaf discs from the misted leaves, anthocyanin content was equal to or greater than in leaf discs from non-misted plants. These results explain why rainfall in autumn reduces the intensity of red foliage coloration in Euonymus and perhaps other ornamental plants by leaching of metabolites, thus delaying the onset of dormancy and leaf senescence and inhibiting anthocyanin synthesis.