Clonal Variation In Rooting Cofactor Content In Rhododendron
From The Rhode Island Nurserymen's Newsletter - June 1968
Choong II Lee, Dept. of Horticulture University of Rhode Island
Reprinted with permission from the Rhode Island Nurserymen's Newsletter
Many kinds of synthetic growth regulators are used in nurseries to stimulate rooting of cuttings. Nurserymen realize that even though cuttings are treated with the known root-promoting substances Rhododendron 'Dr. H. C. Dresselhuys' is very difficult to root, while cuttings of Rhododendron 'Cunningham's White' root very easily. The rooting ability of cuttings also varies from season to season.
It is generally believed in other plant genera that some internal factors (endogenous root-promoting substance), other than auxin, produced in leaves or buds are necessary for root formation of cuttings and the factors are absent or present in insufficient amount in the difficult-to-root cuttings. If the basic reasons for clonal and seasonal differences in rooting capacity of rhododendron cuttings could be learned, production efficiency could be increased. The objectives of some experiments, carried out at the R. I. Agricultural Experiment Station, were to identify the endogenous root-promoting substance in three clones of rhododendron and to study seasonal changes of these substance. If a difficult-to-root clone of rhododendron showed a lack of endogenous root inducing substances, perhaps these compounds should be added and then this clone could be rooted as easily as an easy-to-root clone of rhododendron. Stem tissues of 'Cunningham's White', 'English Roseum', and 'Dr. H. C. Dresselhuys' were collected at seasonal intervals, dried and extracted with methanol. Extracts were separated by paper chromatographic technique and the effect of separated extracts on root initiation of mung bean cuttings was studied.
In every season extracts from 'Cunningham's White' initiated the greatest number of roots in mung bean cuttings, while the least number of roots were developed in the extract from 'Dr. H. C. Dresselhuys'. Extracts from 'English Roseum' showed intermediate effect on root initiation. Results indicate that 'Cunningham's White' contains the greatest amount of endogenous root-promoting substance and the least amount of these substances are present in the difficult-to-root 'Dr. H. C. Dresselhuys'.
The levels of endogenous root-promoting substances generally showed considerable increase in September and decrease again in November to the level of July extracts. Conversely, a rooting inhibitor found in July extracts disappeared in September and reappeared in November extracts. These results may be associated with the fact that better rooting response can be obtained in late summer or fall than in any other season.
The presence of endogenous root-promoting substances and the clonal variation of these substances in amount was also shown by a grafting experiment. Leaf bud scions of 'Cunningham's White' were grafted to 'Dr. Dresselhuys' cuttings. It was found that a leaf and bud scion of 'Cunningham's White' significantly improved both rooting percentage and root ball diameter of 'Dr. H. C. Dresselhuys' cuttings. On the other hand, 'Cunningham's White' showed considerably poorer rooting when a leaf and bud scion of 'Dr. H. C. Dresselhuys' was grafted to the cutting. Rooting capacity of 'English Roseum' was less affected by a leaf and bud scion of other clones of rhododendron.
In conclusion, rooting ability of rhododendron cuttings was found to be related to the level of endogenous root-promoting substances, other than auxin, contained in cuttings. Seasonal variation in rooting response of cuttings may be due to seasonal changes in the level of these endogenous root-promoting substances. Highest activity of these substances was obtained in September.