Effects of growth regulators and decapitation on flowering of Dendrobium orchid hybrids

Plant Science Letters, 12 (1978) 287--292

287

© Elsevier/North-Holland Scientific Publishers Ltd.

EFFECTS OF GROWTH REGULATORS AND DECAPITATION ON FLOWERING OF D E N D R O B I U M ORCHID HYBRIDS

C.J. GOH and A.L. YANG Botany Department, University of Singapore, (Singapore)

(Received November 29th, 1977) (Accepted April 6th, 1978)

SUMMARY The effects of exogenously applied benzyladenine (BA), gibberellic acid (GA) and 3-indoleacetic acid (IAA) on the flowering response of two Dendrobium hybrids, namely, D. Lady Hochoy and D. Buddy Shepler × D. Peggy Shaw, were studied. BA stimulated flowering in mature pseudobulbs; GA was not effective when applied alone but accelerated slightly the BA effect. IAA suppressed the promotive effect of BA. Decapitation of developing or mature pseudobulb stimulated the production of a new pseudobulb from basal node but not flowering. It is suggested that flowering in these Dendrobium hybrids required cytokinins from the roots.

INTRODUCTION In monopodial orchids, it had recently been suggested that flowering was regulated by plant growth regulators, both qualitatively and quantitatively [1]. With Aranda Deborah, auxin inhibited flowering, whereas cytokinin promoted flowering [2,3]. These results were interpreted in terms of apical dominance effect. Previous studies showed that many sympodial orchids responded to short day or low temperature treatment for floral bud initiation [4,5], although daylength treatments did not affect the flowering of some Dendrobium hybrids [6]. Since short day or lower temperature treatment can affect the levels of endogenous growth regulators [7], it appears that the flowering response of sympodial orchids could have been from the change in endogenous hormonal balance. Therefore, the flowering response to exogenous-

288 ly applied growth substances were studied in two sympodial orchid hybrids, Dendrobium Lady Hochoy and D. Buddy Shepler X D. Peggy Shaw. These two hybrids were chosen because the former rarely flowers in cultivation but continues to produce pseudobulbs and the latter is a complicated fifth generation hybrid involving both the Phalaenanthe and Ceratobium sections. MATERIALS AND METHODS

Dendrobium Lady Hochoy (D. Ursula (D. undulatum X D. veratrifolium) X D. taurinum) is a second generation hybrid. D. Buddy Shepler X D. Peggy Shaw is a fifth generation hybrid involving 14 hybridisation crosses among 6 species: D. phalaenopsis, D. bigibburn, D. taurinurn, D. stratioles, D. gouldii and D. undulatum. Among these species, D. phalaenopsis was repeatedly used (8 times} thus the hybrid is similar to D. phalaenopsis in growth habit and flower features. Both these Dendrobium hybrids exhibit characteristic sympodial growth. The pseudobulb continues to develop vegetatively till the apex becomes quiescent. In D. Lady Hochoy, these pseudobulbs may grow to 140 cm with as many as 40 nodes while in D. Buddy Shepler X D. Peggy Shaw, pseudobulbs are mostly about 30 cm in length with 12 to 14 nodes. The new pseudobulb develops from axillary bud of a basal node. Roots are confined to the basal stolonous noaes (pseudobulbs) which form the 'sympodial joints'. Inflorescences when produced develop from the axillary buds proximal to the quiescent apex. Terminal inflorescence was not observed amongst the experimental plants. Well-grown plants in pots obtained from Singapore Botanic Gardens were maintained in the departmental orchid nursery under cool and partially-shaded conditions, a common practice for growing dendrobiums locally. The experimental plants generally received about 3 h of direct sunlight shortly before noon and about 30% of diffused sunlight when shaded. Application of growth substances was by the injection method described earlier [3]. A solution of growth substance was injected by means of a 5 ml hypodermic syringe (with the plunger removed) directly into the internodal (pseudobulb) tissues below the 2nd leaf for the apex in D. Lady Hochoy; or below the 4th leaf in case of D. Buddy Shepler × D. Peggy Shaw. The syringe was left in place and free-flow of solution was maintained by ensuring that the needle was free of loose stem tissues. The solution in the syringe (1 ml) was renewed daffy for a period of 5 days. Control plants were treated similarly with distilled water. Floral bud initiation was considered to have taken place only when the axillary bud broke through the leafsheath and became visible. The response of treated plants were observed for a period of 2]. days for bud initiation and the time taken from first day of treatment was recorded. RESULTS The effects of growth regulators on the mature pseudobulbs where vegetative growth had terminated are summarised in Table I. In Dendrobium Lady

289 TABLE I E F F E C TS OF GROWTH SUBSTANCES ON THE FLOWERING OF DENDROBIUM HYBRIDS Treatment

D. Lady Hochoy Control BA 10 -3 M 1 0 -4 M

GA 10 -3 M I0-' M BA (10 -3 M~ + GA (10 -4 M) BA (10 -3 M) + IAA (10 -s M) D. Buddy Shepler Control BA 10 -3 M BA (10 -3 M) + GA (10 -3 M)

No. of plants treated

Time in days to bud initiation a

5 5

--

5 5 5

7(9), 9(6), 11(4), 12(1), 13(3) 7(5), 9(2), 10(1), 11(2), 13(3) --6(9), 9(2), 11(3),

5

--

5

Total No. of buds initiated

No. of buds developed to maturity

Percentage of plants flowered

0

0

0

23

7

80

13

6

80

0 0 15

0 0 8

0 0 80

0

0

0

0

0

0

5 15

3 6

60 100

13(1)

× D. Peggy S h a w 5 --

5 5

8(3), 9(2) 7(2), 8(12),

9(1)

a Parenthesis indicates the number of buds initiated on that day.

Hochoy, bud initiation occurred in all the plants treated with either BA or (BA + GA). These buds were all produced at the upper terminal portion of the pseudobulb. They were first initiated in about 6 days in plants treated with (BA + GA) and 1 day later in plants treated with BA alone. Some of the initiated buds, usually those initiated earlier, developed to mature inflorescences while others remained dormant or aborted. Plants treated with GA alone did not initiate any buds. Those treated with BA (10 -3 M) together with IAA (10 -s M) developed swollen nodes about 7 days after treatment indicating some growth activities, but these buds did not emerge through the leaf sheaths. During this period, no buds were initiated in the control. These plants remained vegetative for at least 3 months following treatment. With developing pseudobulbs, treatment with BA (10 -3 M and 10 -4 M) did not initiate any buds and the pseudobulbs continued to develop vegetatively. Similar results were obtained with D. Buddy Shepler × D. Peggy Shaw (Table I). While the controls remained inactive, buds were initiatedin mature pseudobulbs treated with B A (10 -3 M), either alone or together with G A (10 -3 M), about 8 days from the firstday of treatment. G A appeared to cause an increase in the number of buds initiatedand also the number of mature inflorescences produced. As with D. Lady Hochoy, buds initiatedearliercontinued develop-

290 ment to maturity while those later ones remained dormant or aborted. To examine the possibility of apical dominance control in the production of axillary inflorescences, further experiments were conducted with D. Lady Hochoy. Decapitation of mature pseudobulbs did not produce any inflorescence, whereas decapitation of currently developing pseudobulbs, ranging from 60 cm--90 cm high (with 25--32 nodes) caused the production of 1 vegetative sideshoot each at the base of the pseudobulb about 20 days after decapitation. These sideshoots continued to develop to mature pseudobulbs. Similarly, total severance of mature pseudobulbs (of 2 growth seasons earlier) from currently growing pseudobulb at the 'sympodial joints' also failed to stimulate floral initiation in the mature pseudobulbs, but sometimes led to the development of a vegetative sideshoot from the basal node. These mature pseudobulbs, however, responded to BA (10 -4 M) treatment and produced inflorescences. DISCUSSION In Dendrobium, flowering does not occur in pseudobulbs which continue to develop vegetatively, but only after the apex has become quiescent. This growth habit indicates the existence of a strong apical dominance effect. In our present experiments, BA treatment with developing pseudobulbs also failed to stimulate floral bud initiation. However, in mature pseudobulbs where vegetative growth had terminated, many pseudobulbs (in D. Lady Hochoy, practically all) still failed to produce inflorescence. This indicates either one or more of the following still operates: A; apical dominance still controlled the outgrowth of lateral inflorescences in these mature pseudobulbs; B: the mature pseudobulbs lacked a promotive factor required for lateral outgrowth; C: presence of other inhibitory growth substances such as abscisic acid to prevent lateral outgrowth; or D: other unknown mechanisms. We have no evidence for the last 2 alternatives and therefore only the first 2 possibilities are discussed below. Presumably, when the apex became quiescent, or following decapitation, the level of auxin in the pseudobulb would decrease. This decrease should release axillary buds from apical dominance. Usually, however, only 1 basal axillary bud developed to form the new pseudobulb whereas all other axillary buds remained inhibited. It may be argued that this developing vegetative bud could produce sufficient auxin which could be transported acropetally into the old pseudobulb to maintain inhibition of lateral bud outgrowth [8]. However, even when acropetal auxin transport occurred, it is unlikely that the young developing new shoot at the base of the pseudobulb could produce sufficient auxin translocated across the 'sympodial joints' to inhibit outgrowth of upper axillary buds of two or more mature pseudobulbs produced in the previous seasons of growth. Total severance of connections from the currently growing pseudobulb also failed to stimulate these upper axillary buds to develop. Moreover, it is noted that those basal axillary buds took around 20 days to break through the leafsheath (very often a longer period was required) whereas

291 bud initiation f r o m the upper axillary buds required only half that time (Table

I). It is therefore unlikely that these upper axillary buds in mature pseudobulbs were still controlled by apical dominance (through auxin levels), although it was shown that 10 -s M IAA could effectively suppress the promotive action of BA at 10 -s M and prevented the buds from breaking through the leafsheaths (Table I). In the present experiments, BA not only stimulated bud initiation but also continued growth of inflorescences to maturity. That cytokinin can effect lateral outgrowth is well documented [8,9] ; together with auxin, it had also been shown to regulate the nature of organogenesis in thin layers of epidermal and subepidermal cells in Nicotiana tobacum [ 10 ]. It seems therefore, cytokinin is required for the production of inflorescences in the Dendrob ium hybrids studied. Since the main source of cytokinin in the pseudobulb appeared to be the roots [11], it could well be that developing vegetative pseudobulbs, or the basal young sideshoots, commanded the supply of the cytokinins synthesized in the roots and thereby deprived the upper axillary buds of the mature pseudobulbs of adequate supply of cytokinins. Consequently, the upper axillary buds in mature pseudobulbs failed to develop. With Aranda hybrids, it was shown that a flowering gradient existed along the stem axis, greatest near the apex and diminished basipetally [12,13]. In the Dendrobium hybrids, it is noted that inflorescences were produced from the upper axillary buds of the pseudobulbs whereas the basal buds developed invariably into vegetative shoots. Recently, it has been reported that in Cymbidium, buds excised from the upper zone of the aerial part of the pseudobulb evolved into well-structured inflorescences in vitro whereas the basal ones developed into vegetative shoots [14]. It is tempting to speculate if flowering gradient also existed among the sympodial orchids. However, more evidence is necessary to substantiate this suggestion. ACKNOWLEDGEMENTS

We thank the Director, Singapore Botanic Gardens, for providing us the plants used in the present study. REFERENCES

1 2 3 4 5 6 7 8 9 10

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