Analysis of Adventitious Root Formation
By: R. L. M. Pierik and H. H. M. Steegmans
Dept. of Horticulture, Agricultural University, Wageningen, The Netherlands
In: Scientia Horticulturae 3(1975):1-20
Reprinted from Ornamentals Northwest, Dr. James Green, Editor
An excellent report on factors affecting root formation in Rhododendron stem explants grown in test tube culture; the author recognizes and discusses the possibility that root formation under the test tube conditions of the experiment is not necessarily comparable with the rooting of cuttings under nursery conditions"...information obtained in tissue culture should be considered an indication of possible processes but should not be regarded as valid until it has been shown to apply to cuttings." Factors enhancing the rooting of stem explants (1.5 cm stem lengths from soft young shoots with green bud scales) included:
1) Removal of a strip of bark the length of the explants strongly enhanced rooting speed, rooting percentage and root dry weight; wounding is a procedure widely practiced in the nursery industry.
2) Oxygen supply to the basal end of the explant was apparently essential. Excessive water supply causes poor rooting in the commercial nursery practices which may be due to lack of aeration.
3) In the experiment, darkness was essential for rooting. Authors conclude that light in both in vivo and in vitro culture has an unfavorable effect on rooting of Rhododendron. While the authors did not explain the action of light in inhibiting rooting of explants or cuttings they made one point of interest: ". . . in nursery practice, the place on the plant from which vegetative cuttings are taken has a considerable effect on rooting ability; cuttings taken from shoots grown in the shade succeed much better than cuttings from shoots grown in full daylight."
4) Optimum temperature for root formation was 25°C.
5) Auxin was an absolute requirement for rooting, the kind and concentration depending on the cultivar used.
6) Root regeneration in the experiment was not influenced by the age of the mother plants nor by the time of the year at which explants were isolated. Authors suggested that reason for this observation in their experiment is that all Rhododendron plants used might have been adult and reacting uniformly. In commercial practice, best rooting has been obtained from cuttings of so-called juvenile plants. In the experiment only morphologically young, soft stems were used since the capacity to form roots almost completely disappeared in stems with increasing shoot age.