Daylength and Temperature in Relation to Flowering Orchids
Gavino Rotor, Jr.

When Garner and Allard pointed out the significance of photoperiodism in 1920, "it was a real surprise to many investigators of plant life that so 'dilute' a factor as day length has so potent an effect on many plants" (Murneek, 1948). Photoperiodism is defined as the response of plants to the daily light period. Garner and Allard demonstrated with Maryland Mammoth tobacco and Biloxi soybean that length of day was the determining factor in the change of the plants from vegetative to flowering condition. When the tobacco plants were exposed to long periods of light daily, vegetative growth continued and the plants finally exceeded 10 feet in height. With short day light periods the plants flowered even when they were less than 5 feet in height. The responses of Biloxi soybean were of a similar nature.

Subsequent studies by the same authors on other species led them to classify plants into four groups on the basis of their response to daylength. (1) Short-day plants as Maryland Mammoth tobacco and Biloxi soybean flower only when the day light period is short. Fall chrysanthemums, poinsettia and cosmos belong to this group. (2) Long-day plants as Centaurea, annual chrysanthemum and nasturtium flower only only with long daily light periods. (3) Indeterminate plants as lupines, roses and Saintpaulia, flower regardless of length of day. (4) Intermediate plants flower only when the daylength was neither short nor long, usually 12-14 hours; above or below this number of hours they remain vegetative.

It is now known that the effects of daylength on the flowering and vegetative growth of plants my be modified or altered by temperature. Greenhouse chrysanthemums will initiate and develop flower buds under short days only when the minimum temperature is not lower than 60 degrees Farenheit; they remain vegetative at a temperature of 55 degrees Farenheit. Other species exhinbit varying responses with different combinations of daylength and temperature.

Results of investigations with many species of plants, including florist crops, have led to the application of the principles of photoperiodism on a commercial scale. To induce flower bud formation in short-day plants when the days are normally long, opaque black cloth is placed about the plants in the late afternoon and removed early in the morning. To prevent short-day plants from developing flower buds when the days are normally short, additional light is given by using ordinary electric bulbs. Lighting plants in the middle of of the night thus breaking the long dark period into 2 short ones is more effective than additional lighting at the end of or just before the normal daylight period (Post, 1942). The use of 40-watt or 60-watt bulbs over the plants to give a minimum light intensity of 5 to 10 foot candles is sufficient to cause photoperiod responses. Post (1949) gives a detailed description of teh proper use of lights and black cloth in timing various florist crops.

Daylenght studies with orchids now under way at Cornell University are concerned with discovering facts that would make planned production possible. Only commercially important genera and species are being used in these studies. These include Cattleya gaskelliana, C. gigas, C. labiata, C. mossiae, C. percivaliana, C. skinneri, C. trianaei, Cymbidium hybrids, Paphiopedilum insigne (commercially known as Cypripedium insigne), Dendrobium nobile, D. phalaenopsis, Phalaenopsis amabilis and P. schilleriana. The plants were obtainedthrough the courtesy of Alberts and Merkel Brothers, Inc. Arcadian Rose Gardens, Beall Greenhouse Company, Edward Manda, Inc., Fight Floral Company, Harry Gray Company, Ringdahl's, L. Sherman Adams Company Orchid Glen, Inc. Patterson and Sons, Thomas Young Orchids, Inc., and Traendly and Schenck.

The experiments involved two lines of investigation: (1) an anatomical study of the initiation of floral primordia and development in each species to determine the normal dates of bud formation and (2) actual treatment of plants under different daily light periods in combination with high and low temperatures.

The daylength treatments were conducted at near 65 and 55 degrees Farenheit minimum temperatures. With both temperatures, long-day conditions were provided by using artificial light, supplementing daylight to give the plants a total of 16 hours of light daily. In the short-day treatments, the plants received only 9 hours of natural light daily. This was acccomplished by completely enclosing the plants withina tent of opaque black cloth. Control plants grown under both temperatures received the normal length of day. The results shown in Table 1 were obtained after one and one-half years of continuous daylength treatment.

The results obtained with Cattleyas up to the present time indicate that C. labiata, C. mossiae, C. percivaliana and C. trianaei are short-day plants. Long days at either 55 or 65 degrees Farenheit prevented flowering in the last three species. With C. mossiae there are indications that, while flower bud intiation takes place under short days, subsequent bud developement may require long days or low temperature. Further studies on this problem are being made. Cattleya labiata was prevented from flowering only with long days at 65 degrees Farenheit. It flowered under other combinations of daylength and temperature used in this study. In 1950, C. percivaliana and C. trianaei plants under short days at 65 degrees flowered 2 to 3 months earlier than those under normal daylength.

Long days at either 55 or 65 degrees hastened the vegetative growth of C. percivaliana, while with C. mossiae and C. trianaei vegetative growth was hastened with long days only at 65 degrees.

Continuous short days at 55 degrees had no effect either on flowering or on development of the vegetative growths of C. gaskelliana, C. labiata, C. mossiae and C. trianaei. However, the vegetative growth of Cattleya percivaliana was delayed and some plants failed to produce either sheathes or flowers. This is commonly referred to as blindness and may result from a lack of carbohydrates due to decreased photosynthesis with the short day.

Short days at 65 degrees also induced blindness in some plants of C. percivaliana, but the vegetative growth was two to three months ahead of plants under normal daylength. Under the same treatment, C. trianei behaved in the same manner.

C. gaskelliana was prevented from flowering only with short days at 65 degrees. Its repsonse to daylength is just the opposite of that shown by C. labiata.

In delaying or advancing the flowering of a species of plants by regulating the length of day, it is necessary to know the normal date of bud initiation. Post (1942) has shown that with some short-day plants at Kalanchoe or Christmas begonia, after the buds are initiated, flowering takes place normally regardless of daylength. But when the plants are given long days with additional light before the normal date of bud initiation, the buds are prevented from forming and the flowering period can be delayed. Conversely, Kalanchoe and Christmas begonia can be flowered early by using black cloth before their normal dates of bud initiation.

The bud initiation dates of the orchid species included in our experiments were determined by microscopical methods. Samples were taken every two weeks, the method of collecting them varying with the morphological characteristics of the species.

Flower buds in Cattleyas are formed at the top of the pseudobulb, slightly above the point of attachment of the leaf. This part was cut off and, after a series of treatments, was sectioned and mounted permanently on glass slides for microscopical examination. The following dates of normal bud initiation of five Cattleya species were determined in 1949 and 1950 in Ithaca.

In the determination of the above dates, it was observed that there was no relationship between the length of the shoot and the time of bud initiation. In Cattleya labiata, for example, most pseudobulbs of the current year's growth were fully mature before June 25, 1949, and yet did not have buds. After this date, however, buds in various stages of developement were found on pracically all pseudobulbs, regardless of whether they were mature or still developing with leaves not yet fully unfolded.

Even mature pseudobulbs (leads) which sometimes developed without flower sheaths, formed and developed flower buds. Also, it is not unusual to find large buds almost ready to open, emerge from developing pseudobulbs when the leaves are still partly unfolded. These two conditins have been observed by orchid growers in their ranges.

The normal date of bud initiation was determined for five Cattleya species.

Bud initiation was found to be related to daylength and not to the length or developement of the pseudobulb or flower sheath.

Cattleya mossiae, C. percivaliana, and C. trianaei were found to require short days for bud initiation under both 55-degree and 65-degree temperatures.

Long days at either 55 degrees or 65 degrees prevented bud initiation in C. mossiae, C. percivaliana, and C. trianaei.

Cattleya labiata was prevented from forming and developing its buds only with long days and a temperature of 65 degrees. It will flower under any daylength at 55 degrees.

The daylength response of Cattleya gaskelliana was opposite to that of C. labiata. It was prevented from flowering only with short days at 65 degrees. It flowered under any daylength at 55 degrees and with long days at 65 degrees.

Long days at either 55 or 65 degrees hastened vegetative growth in C. percivaliana, while in C. mossiae and C. trianaei vegetative growth was hastened with long days only at 65 degrees.

Long days at 55 degrees delayed vegetative growth in C. gaskelliana.

Short days at 65 degrees hastened vegetative growth of Cattleya percivaliana and C. trianaei.

The effects of daylength in combination with 55 or 65 degrees minimum temperatures offer many possibilities in timing the flowering of the five species mentioned.

Literature Cited

1. Garner, W. W. and H. A. Allard. Effect of the relative length of day and night and other factors of the environment on growth and reproduction of plants. Jour. Agr. Res. 18:553-606. 1920.

2. Murneek, A. E. and R. O. Whyte. Vernalization and photoperiodism. A symposium. 196 pp. Chronica Botanica Co., Waltham, Mass. 1948.

3. Post, K. Effect of daylength and temperture on growth and flowering of some florist crops. Cornell University Agr. Exp. Sta. Bull. 787:1-70. 1942.

4. Post, K. Florist crop production and marketing. 891pp Orange Judd Publishing Co., Inc., New York.