QBARS - v7n1 Studies of Factors Inhibiting the Rooting of Rhododendron Cuttings

Studies of Factors Inhibiting the Rooting of Rhododendron Cuttings
By Bernard Thomas Bridgers
Scientific paper No. A384, Contribution No. 2398, of the Maryland Agricultural Experiment Station

Part Two
Combination Wound and Chemical Treatments

ABSTRACT
Factors of inhibition to rooting of rhododendron cuttings were investigated by chemical, environmental, anatomical, and physiological studies.

Tannin analysis were made using various species and varieties, and the rooting response of cuttings subjected to controlled and uncontrolled conditions of temperature, humidity, light, and atmosphere of medium was observed. Cuttings, with and without flower buds and of different ages of growth, were subjected to various wound and chemical treatments. Attempts were made to prevent discoloration at basal ends of cuttings, and cross sections of different ages of stems were observed. Respiration of stem tissue and water absorption of cuttings were studied.

To eliminate the possible inhibitory effect to rooting caused by stem structure, the tissues to the outside of the cambium were interrupted by various methods. Observations from the first experiment indicated that mechanically wounded cuttings without further treatment with root promoting substances rooted slowly as certain catawbiense hybrids treated in such manner showed only slight signs of rooting at end of 12 weeks. Results obtained at the end of 11 weeks after retreatment are shown in Table V.

A marked increase in rooting in both heaviness and percent rooting (shown in Figs. 1 and 2) was noticed when the cuttings were wounded and treated with Hormodin 3 powder. This was in accordance with the work of Kruyt (20) who also observed that the treatment with growth promoting substances gave a marked increase in per cent rooting.

The wound treatment; produced a heavier and a better distributed root system than the unwounded, and extreme basal callusing and subsequent weak rooting was eliminated. Hubert, Rappaport, and Beke (15) have reported in agreement. Data resulting from a study of the influence of age of wood and presence of flower buds on the rooting response of cuttings of R. maximum roseum which were treated with various wound and chemical treatments are shown in Table VI. It was apparent that cuttings taken at the junction of 1 and 2 year wood, with or without flower buds, rooted heavier and a greater per cent rooted than cuttings made of 2 year wood. The rooting response of cuttings taken without flower buds, cut at the junction of 1 and 2 year growth, subjected to the stripped wound treatment, and with or without the wax dip, was greater than similar cuttings treated in the same manner but with flower buds. Working with various rhododendron species, Kemp (17) also reported in agreement that shoots with vegetative buds were superior in rooting to those with flower buds. The apparent detrimental effect of flower buds on root development may be seen in Fig. 3.

When cuttings without flower buds, taken at the junction of 1 and 2 year wood, were subjected to Hormodin 3 in combination with the stripped wound treatment, with or without the wax treatment, there was an increase in both heaviness and per cent rooting. Furthermore, there was a similar increase in response obtained when cuttings made in the same manner but treated with a combination of Hormodin 3 and Fermate, prepared in a 3 to 1 ratio, with or without wax. There was a decided increase in average rooting score when Fermate and wax were used as compared to Hormodin 3 treatment alone.

A study was made to compare various wound and chemical treatments on the rooting response in percent rooting and average rooting score of cuttings of R. 'Watereri'. From the data presented in Table VII and the analysis in Table VIII, the stripped and sliced wound methods were significantly better than the slit. There was no significant difference between the chemical treatments, but each of these treatments was highly significant to the untreated.

When heaviness of rooting was considered, (Tables IX and X.) the stripped and sliced wound methods were noted to be highly significant to the slit. Furthermore, a study of the various chemical treatments showed that Hormodin 3 and Fermate, at a ratio of 3 to 1, with the wax was highly significant to all other treatments. Treatment by solution immersion for 24 hours using indolebutyric acid at a concentration of 100 milligrams per liter was highly significant to the same treatment with wax. Results from all chemical treatments were highly significant to those of the untreated cuttings.

In the interaction of methods of wounding and chemical treatments, there was noted that with the stripped and sliced methods, Hormodin 3 plus Fermate, with or without wax, was highly significant to the same chemical treatment when used with the slit method.

A comparison of early root development of the various wound and chemical treatments may be seen in Figs. 4, 5 and 6. In the majority of cuttings observed, the chemical treatment using Hormodin 3 and Fermate (3:1) with wax was found to produce roots earlier than the other treatments. This was believed to be the explanation as to the reason this treatment was found significantly better, when heaviness of rooting was considered, than the other treatments.

A further comparison of the sliced and stripped wound treatments with the various chemical treatments was studied using cuttings of R. catawbiense 'Roseum Elegans'. Data, based on per cent rooting, (Tables XI and XII) showed the sliced method was significant to the stripped.

There was no significant difference between the Hormodin 3 and Fermate when used with or without the wax: however, these treatmnts were highly significant to treatments using indolebutyric acid, either alone or with wax.

Considering the rooting response in terms of heaviness of rooting, (Figs. 5 and 6) the sliced wound method was highly significant to the stripped. In using wax with Hormodin 3 and Fermate, there was a significant difference over the unwaxed, and both of these treatments were highly significant to the indolebutyric acid treatments. There was no significant difference between the untreated cuttings and those treated with indolebutyric acid.

From the results it was concluded that the stripped and sliced wound methods were superior to the slit, and the sliced method was superior to the stripped. Chemically treating with Hormodin 3 and Fermate (3:1) and using the wax dip appeared to be the best treatment.

Apparently there was a toxic effect to cuttings when treated by solution immersion for 24 hours with indolebutyric acid at 100 milligrams per liter. Cuttings of R. catawbiense 'Roseum Elegans' showed approximately 90 per cent fatality when treated in this manner. An example, of this toxicity may be seen in Fig. 7. Similar observations (15) were reported concerning wounded cuttings of Prunus sinensis.

PHYSIOLOGICAL STUDIES

Wounding of the stem by different methods has appeared throughout the investigation to be beneficial in both heaviness and per cent rooting, and the wax treatments have been advantageous in like manner. Certain physiological studies were made in order to obtain a possible explanation for the behavior of these treatments.

A respiration study of excised portions of the stems, as shown in Figure 8, revealed that wounding greatly increased respiration. The stripped method gave the greatest increase. and the sliced method showed a decided increase over the slit.

When the stem portions were completely waxed (Fig. 9) there was a definite decrease in respiration, which is in agreement with Platenius (22) who used "Brytene" on various vegetables. The data, however, showed the same general trend of respiration in that unwounded tissue used less oxygen, followed by wound methods of slit, sliced, and stripped in increasing order.

The increase in respiration caused by wounding was believed to be explained by the work of Wiesner, as reported by Bonner and English (4), which showed that wounding plant tissue caused a resumption of meristematic activity of cells apparently mature. They concluded that when plant tissue was injured substances were formed or liberated which were capable of causing adjacent uninjured cells to resume greater activity.

Day (8) showed that wounding cuttings of Bartlett pear increased water absorption and postulated that this might be a reason for an increase in rooting response. An increase, as shown in Table XV, was also observed with wounded cuttings of R. maximum. When cuttings, with or without wax, were wounded by the slit method there was an increase in moisture uptake over the unwounded; and both the sliced and stripped treatments, with or without wax, showed a decided gain over the slit. The wax treatments showed no appreciable effect.

Interrupting the epidermal layer, previously described, and exposing tissues of the cambium region to direct contact with water was believed to be an explanation for these observations. Since the wax was water soluble its effect on water uptake would appear to be negligible.

It was believed by the author that increased respiration which resulted from wounding the cutting brought about greater activity in the meristematic tissue, and increased water absorption due to wounding was beneficial in maintaining turgidity in the cutting. Therefore, the beneficial effects in rooting response obtained by wounding the cuttings might be explained by increased respiration and water absorption.

An explanation for the increase in heaviness of rooting often noted by the wax treatment was not offered.

SUMMARY AND CONCLUSIONS

To determine the causal factors inhibiting the rooting of rhododendron cuttings various chemical, environmental, anatomical, and physiological studies were performed.

Component stem parts of various species and varieties were quantitatively analyzed for tannins, and a possible correlation between ease of rooting and tannin content was investigated. Attempts were made to prevent the discoloration at basal ends of cuttings by means of treatments using citric acid and wax.

Cuttings were rooted under changing and controlled conditions of temperature, humidity, and light. In another investigation the atmosphere of the rooting medium was varied by using normal, high, and low oxygen concentrations.

Cross sections of stems were observed and a comparison between the structure of 1 year and other ages of growth was made.

Based on the foregoing study, experiments were performed in which cuttings of various hybrids, with and without flower buds and of different ages of growth, were subjected to various wound treatments and these treatments in combination with various chemical treatments.

Physiological studies consisted of measurement of respiration of excised d portions of stems and water absorption of wounded and waxed cuttings.

From the results of these investigations the following conclusions may be made.

1. The greatest concentration of tannins was found in the 1 year old stem, followed, in descending order, by bark of 1 year stem, leaf, 2 year stem, petiole, 2 year bark, flower and vegetative buds, and wood. Furthermore, there was no correlation observed between tannin content of the cuttings and ease of rooting.
2. There were no consistent differences in rooting response when cuttings were subjected to controlled or uncontrolled conditions of temperature, humidity, and light. Increasing oxygen concentration of the atmosphere in the rooting medium resulted in heavier root systems and higher per cent rooting than when normal and low oxygen concentrations were used; low oxygen concentration definitely increased rooting response.
3. Differences in structure of 1 year and older stems were not observed except the width of tissues from the epidermis to the cambium decreased with age. The heavily cutinized epidermal layer was believed to be a possible inhibitory factor in rooting.
4. Rooting response was greater when cutting 3 were taken at the junction of 1 and 2 year wood as compared to 2 year wood. Flower buds on cuttings seemed to inhibit rooting.
5. Discoloration at the basal ends of cuttings was higher in wounded cuttings than in unwounded. Treatment with citric acid and wax was inconsistent in prevention.
6. The stripped and sliced methods of wounding produced a greater rooting response than the slit method.
7. Root promoting substances greatly increased the rooting response of cuttings. A combination of Hormodin 3 and Fermate, ratio 3 to 1, gave an increase in both heaviness and per cent rooting as compared to Hormodin 3 alone; there was an additional increase in heaviness of rooting when wax was used in combination with Hormodin 3 and Fermate.
8. Treatment of cuttings with Hormodin 3 and Fermate, ratio 3 to 1, produced a greater rooting response in both heaviness and per cent rooting than when indolebutyric acid, solution immersion treatment for 24 hours with 100 milligrams per liter, was used.
9. Respiration of stem tissue was increased by wound treatments.
10. Cuttings subjected to wound treatments absorbed more water than unwounded cuttings.
11. The greatest rooting response in terms of both heaviness and per cent rooting was obtained when cuttings of 1 year wood without flower buds were subjected to the sliced wound treatment, treated with Hormodin 3 and Fermate (3:1), then waxed, and rooted in a medium of half sand and half peat with a normal atmosphere and under uncontrolled conditions of light, temperature, and humidity.

From the data the following conclusions may be drawn:

1. No correlation was found between tannin content and ease of rooting.
2. Enriched oxygen content of rooting medium improved rooting response; however, no consistent differences were observed when cuttings were subjected to controlled and uncontrolled light, temperature, and humidity.
3. Stem anatomy showed a heavily cutinized epidermal layer.
4. Flower buds and 2 year wood appeared detrimental to rooting.
5. Discoloration at the basal ends of cuttings was higher in wounded cuttings than in unwounded.
6. Stripped and sliced wound methods produced a greater rooting response than the slit method.
7. Root promoting substances increased rooting response. Hormodin 3 and Fermate (3:1) appeared to give better results than Hormodin 3 alone; when these treatments were used with wax, an increase in heaviness of rooting resulted.
8. Respiration of stem tissue was increased by wound treatments.
9. Wounded cuttings absorbed more water than unwounded cuttings.

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