Sunday, March 05, 2006

Flowering of Orchids



Flowering of Orchids



Written by [Philippine Orchid Review] GOH CHONG JIN
Sunday, 01 June 1986
THE ORCHID FAMILY is one of the largest families in the plant kingdom. It consists of about 600-800 genera with a total of 20,000 to 30,000 species (Garay, 1960; Schultres and Pease, 1963). Orchids are widely distributed in all parts of the world except the extreme cold regions where no flowering platns can survive. They are found most abundantly in the dense tropical forests, but can also be found on open grasslands, hto and dry deserts, cold and damp rocks subject to constant sea spray (Arditti, 1979), and even subterranean, underground, as in Rhizanthella gardneri, a monotypic genus from Western Australia (Nicholls, 1969).
All of us who are familiar with orchids are aware that many of them have definite flowering seasons. This is more pronounced in species and hybrids from temperate rather than tropical regions. Amongst the topical orchids, many have peak flowering periods even if they flower throughout the year. Many keen orchid observers have reported the flowering dates and periodicity of orchids under natural habitats. For example, Curtis (1954) recorded the annual fluctuation in rate of flower production by native cypripediums; Dunsterville and Dunsterville (1967) reported on the flowering season of Venezuelan orchids and Quisumbing (1968) on the Philippines orchids. The most extensive observations were carried out in West Africa by Sanford (1971). All these observations showed presence of flowering seasons and point to the fact that flowering in orchids is regulated by some environmental factors.
Two important factors which control flowering are light and temperature. The effect of light is not obvious in the phenomenon known as photoperiodism, but light intensity and light quality are not without effects. Temperature effects are expressed in vernalisation and thermoperiodism.
Photoperiodism
Daylength varies with the season in most part of the world. Days become longer in the spring following the shortest day (December 21st in the Northern Hemisphere) in the winter. They reach a maximum during the summer (June 21th in the north) and then decrease again in the autumn. The variations in daylength are very small near the equator (about 10 minutes in Singapore – Holttum, 1953), and increase towards the poles. This means that orchids growing in different regions are subject to different daylengths or photoperiods which vary with the seasons.
There are basically three types of responses to photo9periods: long day plants. Long day plants are those which flower (or flower more profusely) when the daylength exceeds a certain minimum (called critical daylength). Short day plants flower when the daylength is shorter than a certain maximum. The day neutral plants flower irrespective of the daylength, that is, they are indifferent to day-length, but may be controlled by some other factors.
It has to be emphasized that long day or short day does not mean longer or shorter than 12 hours of light respectively. A long day plant may flower under 10 hour day or a short day plant may flower in 13 hour days. However, in the case of the 10-hour (critical period) long day plant, as the day length increases, the flowering process is greatly stimulated and accelerated, and it will not flower at all if the daylength is shorter than 10 hours. The reverse is true for the short day plants. In nature, long day plants flower on the increasing daylength of spring and summer. Short day plants flower with decreasing day length in the autumn.
Although photoperiodic effects on flowering was well established it he 1920’s, studies in orchids apparently did not commence until the 50’s. Gavino Rotor Jr. was one of the early researchers in of two weeks (Goh, 1977; 1979; Goh and Yang, 1978;) on Cymbidium, Dendrobium, Paphiopedilum and Phalaenopsis when he was a research assistant at Cornell University, Ithaca (Rotor, 1952). Until today, only a few studies have been reported and the number of species or hybrids examined is extremely small. Some of the orchids with known photoperiodic responses are listed in Table 1. It may be noted that some of those listed are deductions from flowering dates under natural environments (Sanford, 1971). In some studies (e.g. Bhattacharjee, 1979), plants were subjected to continuous long day or short day treatments throughout the whole year. These observations would require further confirmation. In many other plant with photoperiodic requirements, only a few cycles are needed to evoke the flowering process: our studies with Aranda as well as Dendrobium hybrids showed that floral bud initiation can be effected within a period of 2 weeks (Goh 1977; 1979; Goh and Yang, 1978). Therefore, orchids which appeared to require long periods of day-length treatments for bud initiation could be regulated by some other (or additional) factors.
As mentioned earlier, most of the orchids which respond to short-day or long-day photoperiods are temperate in origin. The claim that “… tropical (orchid) plants are more sensitive to small differences in daylength than are temperate-zone plants” (Sanford, 1974) needs further study.
In the greenhouse, photoperiods can be extended by the provision of artificial illumination (incandescent or fluorescent lamps) at the end fo the day, or by breaking long nights into two short night periods with an half hour artificial illumination in the middle. Conversely, days can be shortened by covering the plants with black cloth during part of the day. Practical growers can use the photoperiodic requirements of orchids to regulate blooming time, by either inducing flower production with appropriate photoperiods or delayi8ng it with unfavorable daylengths. For example, Cattleya gaskelliana when grown at 65 oC (18 oC) is a long day plant with a critical daylength of 16 hours. It also requires three to four months from bud initiation to flowering. So, when grown at 65 oF (18 oC) these plants should be given 16-hours days (or long nights with a half hour light break in the middle), three-four months before the flowers are needed. On the other hand, Cattleya labiata (and its hybrids) flowers on short days (critical daylength 16-1/2 hours.) Therefore, to obtain flowers for Christmas, the plants should be kept at 65 oF (18 oC) under long days (or given a half hour light break at the middle of long nights) until September 25th (or round-about) and then kept under daylength of 16 hours or less.
Table1. Examples of orchids with known photoperiodic responsesLong Day Plants Short Day Plants Day Neutral Plants Aerangis biloba* Brassavola nodusa Arachnis Maggie Omei Cattleya gaskelliana Cattleya amabilis Aranda Deborah Cyrtorchis hamata* Cattleya labiata Aranda Wendy ScottDiaphananthe curvata* Cattleya mossiae Cattleya Enid Eulophia guineensis* Cattleya trianae Dendrobium Jaquelyn ThomasLaelia purpurata Cattleya Bow Bells Dendrobium Lady Fay Miltonia anceps Cattleya Jean Barrow Paphiopedilum insigne Miltonia spectabilis Dendrobium phalaenopsis PhalaenopsisOdontoglossum bictonense Phalaenopsis amabilis Vanda Miss JoaquimPolystachya modesta* Renanthera imschootiana Rhyncohostylis gigantea
* Probably, deduction from flowering dates in nature (Sanford, 1971). For details, see Goh, Strauss and Arditti (1982).
Light Intensity
Many tropical orchids are day neutral plants,. For example, Vanda Miss Joaquim. However, they also exhibit peak flowering seasons. In Hawaii, the seasonal flowering of Vanda Miss Joaquim was shown to be correlated with sunlight availability. Reduction in sunlight by the use of saran-cloth cover resulted in progressively later, commencement of flowering and lower yields (Murashige, Kamemoto and Sheehan, 1967). Similarly, when the plants were exposed to varying periods of direct sunlight in Singapore, it was observed that inflorescence production was dependent on the length of exposure to direct sunlight; those plants exposed longest flowered most profusely while those exposed to only four hours of direct sunlight daily did not flower at all (Goh and Wan, 1974).
Many Aranda hybrids are known to behave in the same manner. These plants require full sun for flowering and any shading would delay or suppress the process. In Aranda Wendy Scott, plants which received only three hours of direct sunlight (with eight hours of diffused and reflected light) remained vegetative, whereas those exposed for eight fours of direct sunlight (with three hours of shaded light) produced inflorescences regularly. When the former were transferred to eight-hour direct sun exposure condition, all the plants produced visible floral buds in seven to ten days and practically all the buds continued to develop to mature inflorescence (Table 2.)
The effect of light intensity on the flowering of shade-loving orchids is less obvious. These plants, such as Dendrobiums, Oncidiums, and Phalaenopsis, would not be able to tolerate direct exposure to tropical full sun, they would be scorched within hours if exposed to the strong midday sun directly. Even under partial-shade condition (with overhead green netting), the flowering behavior of Oncidium Goldiana was shown to have a ‘negative correlation’ with the sunshine hours two months before harvest by Pearson’s method (Ding, Ong and Yong, 1980).
Table 2. Effects of direct sunlight exporesure period on the flowering of Aranda Wendy Scott.
Direct Exposure Period
No. of Plants Treated
No. of Plansts Responded
No. of Florail Initials Produced
No. of Inflorescence Developed to Maturity
Experiment 1




3 hours
9
3
3
0
8 hours
8
8
8
7
Experiment 2




3 hours
10
3
3
0
8 hours
10
10
11
10
During the one month period, Floral initials in plants exposed for eight hours were produced within ten days after transfer from shaded conditions.
Temperature Effects
Low temperature has been shown to induce flowering in Cymbidium species and hybrids. Similarly, Phalaenopsis schilleriana requires a period of low temperature for floral induction (Rotor, 1952; de Vries, 1953). These are examples of vernalisation effect.
The effect of low temperature is also well known in another tropical orchid, Dendrobium crumenatum, the pigeon orchid. These plants produce inflorescences at intervals and flower buds are differentiated successively at the nodes of the inflorescence axis. However, the flower buds remain dormant at a very early stage of development. They start to develop again after a period of low temperature and bloom nine days later. This phenomenon, which can be described as thermoperiodism, has been examined for nearly a hundred years but the detailed physiological and bio-chemical processes involved are yet to be unraveled. It may be seen that low temperature may affect not only flower initiation but also flower development.
The periodic response of some orchids may be modified by temperature. For example, Cattleya gaskelliana grown at 18 oC (65 oF) flowered under long-day but not under short-day conditions; however, at 13 oC, they flowered under both long day and short day tr4eatments (Rotor, 1950; 1952).
Table 3. Some orchids requiring low temperature stimulus for flowering
Broimheadia finlaysoniana
Dendrobium Phalaenopsis
Cattleya Mosimae
Odontoglossum Citrosmum
Cymbidium
Paphiopedilum Insigne
Dendrobium Angulatum
Phalaenopsis Schilleriana
Dendrobium Dendrocola
Sarcochilus Appendiculatus
Dendrobium Insigne
Thrixspernum Calceolus
Dendrobium Nobile

Some examples of orchids which require long temperature stimulus for flowering are listed in Table 3. The notable ones are the Dendrobiums and the cymbidiums. Indeed, Coster (1926) listed 15 species of Dendrobium which flowered eight days after the cold temperature experience; eight species including D. crumenatum after nine days; six species after ten days and three species behave the same way.
Hormonal Control
It is increasingly evident that environmental effects on plant growth and development are mediated through the changes in the levels of endogenous plant growth regulators (hormones). With Vanda Miss. Joaquim, it was found that the flowering intensity was inversely correlated to the auxin level in the shoot apex; plants that were exposed longest to direct sunlight and flowered most profusely had the lowest level of auxin, whereas those given only four hours of direct sunlight remained vegetative and had the highest level of auxin (Goh and Wan 1974).
In Aranda Deborah, decapitation induced flowering, but this response could be inhibited by a continuous supply of exogenous auxin (Goh and Seetoch, 1973) It was therefore suggested that the potential of flowering was present at all times. However, its expression was regulated by the endogenous auyxin level in the shoot apex. In other words, flowering is regulated by the correlative apical dominance effect. This suggestion is supported by the fact that application of a cytokinin (benzyladenine) caused production of multiple inflorescences. Similarly, anti-auxins and growth retardants were effective to stimulate flowering (Goh, 1977). These results indicate that auxin inhibits flowering in monopodial orchids.
In fact, the involvement of auxin in the control of flowering in orchid was suggested as early as 1953. With Phalaenopsis schilleriana, de Vries (1953) observed that plants sprayed with auxin solutions tend to be less abundant in flowering and suggested that there seemed to be an excess auxcin which inhibited the production of inflorescence under conditions in Bogor, and that the low temperature acted upon “an auxin controlling factor.”
Apart from auxins, other known growth substances are also involved. In our experiments with A. Deborah, high concentration of gibberellins (GA3, 10-4M) could cause the normal vegetative shoot apex to be transformed into reproductive growth to become a terminal inflorescence (Goh, 1979a). These reversions of vegetative growth and reproductive growths have been observed earlier in other monopodial orchids Arachnis Maggie Oei and Aranda Queen of Purples (Goh, 1976). Thus gibberellins may also have the role in flowering.
When the apices of Aranda Wendy Scott, a hybrid requiring full sun for flowering, were allowed to diffuse on agar for seven and a-half hours under the sun or shade conditions, no great differences in the amounts of diffusible auxins were observed, but the presence of inhibitors(s) were indicated under the shaded conditions by the Avena coleoptile straight growth method. The experiment was repeated with shoot apices from plants grown under different conditions: (a) sun, (b) transferred from shad to sun for 5 days and (c) shade. Three shoots (with seven leaves each) were tested in each sample and the diffusion period was seven hours. The results showed that more auxins diffused from shoots grown I the sun that that from the shade plant. The amount of auxin diffused from plants which were transferred from shade to sun remained low. Extractable auxins (after diffusion) were higher in shade plants compared to sun plants. On the other hand, diffusion or trans-location of inhibitor(s) appeared to occur faster (or greater) in the shade palnts, although inhibitor(s) was (were) extracted from all the samples. The diffusible gibberellins content from plants growtn under sun was much lower than that from plants grown under shade, whereas the reverse was true for extractable gibberellins extracted from the shoots after diffusion on agar.
While the bioassay showed changes in the level of endogenous growth substances occurred under different sun and shade conditions, it also indicated another possible mechanism in the control of flowering: inhibition by growth inhibitor(s) . It is likely that the inhibitor(s) contained at least in part, abscissic acid (ABA) which has a RF value of 0.5-0.7. Indeed, exogenous application of ABA to decapitated A./ Deborah inhibited the production of inflorescences (Goh, unpublished data). However, further studies are required to establish the role of ABA in the flowering of monopodial orchids.
With the sympodial orchids, our studies have shown that cytokinin can also induce flowering in Dendrobium hybrids, for example, D. Louise; D. Lady Hochoy and D. Buddy Shepler x D,. Peggy Shaw. The cytokinin effect is enhanced by simultaneous application of gibberellin (Goh, 1979; Goh and Yang, 1978). In these hybrids, however, auxin and consequently apical dominance effect are not likely to be involved since flowering can only occur in mature pseudobulbs which have completed their vegetative growth, their apices become dormant, and yet many mature pseudobulbs do not flower. Therefore, the requirement of cytokinin (and perhaps gibberellin too) may be involved more directly in the control of vegetative and reproductive development in these orchids.
Although it is believed that the environmental effects are mediated though changes in levels of endogenous growth substances, repeated attempts in our laboratory to supply Dendrobium crumenatum failed to stimulate further development of the floral buds; these buds responded subsequently to a cold night temperature treatment and bloomed nine days later. Thus, thermoperiodic control on floral development could involve more than changes, fir example, an increase in gibberellin level.
Gibberellins have also been reported to sitmulate flowering in Bletila striata (Sano et al, 1961), Cymbidium (Bivins, 1968, 1970) as well as Zygopetalum (O’Neill, 1958).
Flowering Gradient
Many Aranda hybrids, including A. Deborah, A. Hilda Galistan, A. Lusy Laycocok, A. Mei Ling and A. Nancy, exhibit a flowering gradient along the stem axis (Goh, 1975; 1977). In these hybrids when the axillary buds were stimulated to develop by decapitation, those near to the apex developed into inflorescens while the lower ones away from the apex developed into vegetative shoots, indicating that htflowering capacity was greatest near the apex and diminished basipetally.
Depending on the level of the decapitation, one or two seasons of response may be obtained. When the cut was near to the apex, the plants produced either inflorescence or a mixture of inflorescences then a second season of development occurred after the maturation of these inflorescence which took about seven months. The second response produced either a mixture of vegetative shoots and inflorescences or vegetative shoots only. When the cut was low, only vegetative shots were produced. Experiments with fully grown plants of about two meters in height showed that in A./ Lucy Laycock as well as A. Mei Ling, decapitation at 15th or 16th node (counting from shoot apex basipetally) still caused the production of inflorescences; in A. Hilda Galistan and A. Nancy, this level was found to be around 18th to 20th node. In A. Deborah, inflorescences were still produced when decapitated at the 25th node (Goh, 1975; 1977).
These observations had recently been extended to A. Pieter Ewart, A. Christine No. 1 (Koay and Chua, 1979) as well as Aranthera James Storie and Holttumara Maggie Mason (Koay and Chua, 1980). Since this orchids are in good demand in the cut flower trade, theabove flowering characteristics may be made use of in the commercial production of cut lfoewrs. In practice, these orchid hybrids need only be decapitated as close to the apex as possible and flowering will occur. This decapitation method is essentially for a once-a-year crop. However, continual production is not only possible but feasible in a large nursery where plots of plants can be rotated in such a way that inflorescence production can be scheduled on a monthly basis or to meet seasonal demands. Practical details and laternate methods have been discussed earlier (Goh, 1978).
Concluding Remarks
In view of the fact that Orchidaceae is not only large in number of species (and there are thousands and thousands of hybrids), but also wide ecological distribution, it is not surprising that the factors controlling flowering are so variable. The available information is still very limited compared with those on some other horticultural crops like chrysanthemums or tulips. Nevertheless, it is seen that a certain degree of control is possible, at least in some of the commercially important species or hybrids. More work on the physiological aspets would certainly enhance our understanding and perhaps in the not too distant future, we will be able to regulate the flowering or more orchids.
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