Light Duration

J. Raymond Kessler, Jr.


Duration of Irradiation

      Duration refers to the period of time in 24 hours that plants are exposed to light. In temperate regions where greenhouse crops are grown, day length changes seasonally. This change results occurs because the earth's axis is tilted 23½ degrees from a line perpendicular to the plane of the earth's orbit about the sun.

      In the summer in the northern hemisphere, North American is tilted toward the sun so the days are longer than the nights. In the winter, North America is tilted away from the sun so the days are shorter than the nights. At the equator, the day length is relative constant at 12 hours and 7 minutes during the year. As the distance from the equator increases (north latitude), day lengths are longer in the summer and shorter in the winter. The longest day of the year is the summer solstice (≃ June 21) and the shortest is the winter solstice (≃ December 21). Day and night lengths are equal on the autumnal equinox (≃ September 21) and vernal equinox (≃ March 21). When considering day length, weather services report sunrise to sunset, however many plants can perceive twilight. So day length for plants is sunrise to sunset plus twilight.



      While working at the USDA Plant Industry Station, Beltsville, MD early in the 1920's, W.W. Garner and H.A. Allard were the first to recognize the role of light duration on flowering in plants. They wondered why "Maryland Mammoth" tobacco failed to flower in the summer even thought it grew to ten feet, but flowered in the greenhouse in winter at less than five feet. Then also wondered why certain varieties of soybean when planted at two week intervals from May through July all flowers at the same time. After studying irradiance, temperature, soil moisture, and nutrition, they discovered that it was the length of the day that was influencing flowering. this was accomplished by extending the day using electric lamps or shortening it by placing plants in light-tight cabinets.

      Garner and Allard named this response of plants to day length Photoperiodism. After a study on numerous plants, classified plants into three categories based on day length response for flowering: 1) short-day plants, 2) long-day plants, and 3) day-neutral plants. Unfortunately, Hamner and Bonner in 1938 were able to show that it is not the length of the day that plants are perceiving, but the length of the dark period. So a short-day plant should be called a long-night plant, but the earlier terminology persists. Classification of plants based on response to day length for flowering:


Short-Day Plants - plants that initiate flower buds when the day length is shorter than the critical day length or when the night length is longer than the critical night length. Day lengths longer than critical result in vegetative growth.


Long-Day Plants - plants that initiate flower buds when the day length is longer than the critical day length or the night length is shorted than the critical night length. Day lengths shorter than critical result in vegetative growth.


Day-Neutral Plants - plants that initiate flower buds over a wide range of photoperiods. Generally, these plants form flower buds after the plant reaches specific size or stage of development.


Short-Long Day Plants - plants that initiate flower buds only when a sequence of short days are followed by long days.


Long-Short Day Plants - plants that initiate flower buds only when a sequence of long days is followed by short days.


      One important point about these classifications is that there is no mention of the number hours of light that constitutes a long or short day. Only that the day length must be longer or shorter than a critical day-length for the plant in question to flower. Each plant has its own specific critical day length!

      It is even possible for a short-day and long-day plant to flower at the same photoperiod. Example, Cocklebur are short-day plants with a critical day length of 15 hours, it flowers at day lengths less than the critical. Spinach is long day plants with a critical day length of 13 hours, they flower when the day length is longer than critical. A day length greater than 13 and less than 15 hours will cause both plants to flower.

      In the five classes of plants above, the response to photoperiod is qualitative or absolute. A qualitative short-day plant has an absolute requirement for short days or it will not flower. Qualitative long-day plants require long days to flower. Other plants respond to day length in a quantitative manner. They do not require a specific photoperiod to flower and will eventually flower regardless of photoperiod. However, an inductive photoperiod will speed flowering, e.g. petunia.


Photoinductive Cycles

      The number of photoinductive cycles required for floral initiation varies with the species and cultivar:


Poinsettia initiates flower buds in 7 to 20 short days depending on the temperature but must receive short days for at least 40 successive 24 hours cycles before exposure to long days without interference in flower and bract development.


Kalanchoe blossfeldiana requires at least 21 days to initiate flower buds. Fewer short days causes the formation of a vegetative inflorescence. After four weeks of short days, plants can be placed under long days and flowers will develop normally.


Cocklebur requires only one short day for complete flowering.


      Once photoperiodic plants are placed under inductive conditions, persistent changes occur in the growing apex of the plant. The apical meristem changes organization from producing stems and leaves to producing a floral structure. This process is called Photoperiodic or Floral Induction.

      Studies in which various parts of the plant were cover with a black hood to simulate short-days for that particular part while the rest of the plant was under long-days have shown that the uppermost maturing leaves are the sites of photoperiodic perception.



      Flowering in photoperiodic plants is sensed and regulated by a pigment known as Phytochrome. Phytochrome exists in plants in two inter-convertible forms. One form (Pr660) absorbs light in the red (660 nm) portion of the light spectrum and is converted to the second form (Pfr730) that absorbs light in the far-red portion of the light spectrum. Exposure of Pfr730 to far-red light converts it back to Pr660. In addition, Pfr730 slowly reverts back to Pr660 in darkness.

      Pfr730 is the physiologically active form of phytochrome and inhibits flowering in short-day plants and promotes flowering in long-day plants. There is a critical Pfr730 level below which flowering is no longer inhibited in short-day plants and flowering is no longer promoted in long-day plants. With respect to phytochrome conversion, sunlight functions as red light, therefore, Pfr730 predominates at the beginning of the dark period. So the Pfr730 form is rapidly produced in the light and the Pr form is slowly produced in darkness. Under the long days of summer, the nights are not sufficiently long enough to allow enough Pfr730 to revert to Pr660 so the level Pfr730 does not drop below the critical level. Therefore, short-day plants do not flower and long-day plants flower. During the short days of winter, the nights are long enough for sufficient quantities of Pfr730 to revert to Pr660 so that the level pf Pfr730 does drop below the critical level. Therefore, short-day plants flower and long-day plants grow vegetatively.


Night Interruption

      Interruption of the dark period during short days (long nights) with red light stops the reversion of Pfr730 to Pr660 and converts Pr660 to Pfr730. This prevent the level of Pfr730 from dropping below the critical Pfr730 level and, therefore, inhibits flowering in short-day plants and promotes flowering in long-day plants.

      Armed with this understanding, equipment and procedures have been developed to manipulate photoperiod so that photoperiodic plants can be flowered or kept vegetative at any time during the year. Probably the biggest commercial success is with the short-day plant, chrysanthemum. Potted chrysanthemums are grown and sold in large number on a 52 week a year production schedule by manipulating photoperiod.

      Photosynthesis requires high light intensities for growth. In contrast, photoperiodism is controlled by very low light intensities and short periods of lighting. When natural photoperiods are short, night interruption lighting (night-break lighting) is used to promote flowering in long day plants and prevent flowering in short day plants. This is commonly accomplished by supplying red light from incandescent lamps. Incandescent lamps are far more frequently used for night interrupted lighting than other lamp types because they are high in the red wavelengths and inexpensive to install and maintain. More recently, growers have started using some of the high intensity discharge type lamps such as metal halid or high-pressure sodium lamps. These lamps are far more efficient than incandescent and can also be used for supplemental lighting (see the next lecture).

      The general recommendation calls for a minimum of 7 to 10 ft-c at plant height along the edge of the bench. Night interrupted lighted is generally applied from 10:00 PM to 2:00 AM unless otherwise specified for a particular crop. Light intensity for a particular installation can be checked using a light meter at night. Be sure to check the light intensity for the plants furtherest from the lamps. Suitable reflectors or lamps with internal reflectors will increase the light efficiency.

      General design: For a 4 foot wide bench, one string of 60 watt, incandescant bulbs 4 feet apart should be installed no more than 5 feet above the bench surface along the middle of the bench. A wider bench (up to 8 feet) can be lighted with one string of 100 watt bulbs 6 feet apart and no more than 6 feet above the bench. Larger lamps can be used for bigger installations based on a minimum of 1½ watts per square foot.

      Interrupting the middle of the night period has been found to be more efficient than extending the light period by adding light at the end or beginning of the day. Figuratively, the plant thinks it has been through two short nights. The standard program calls for four hours of incandescent light applied in the middle of the night period. Therefore, the lights should come on at 10:00 PM and be turned off at 2:00 AM. Standard greenhouse time clocks that operate on a 24 hour basis are usually used.


Cyclic Lighting

      This technique uses a series of short alternating light and dark cycles to substitute for one continuous light break. The main reason for cyclic lighting is to reduce energy cost required for large installations because all the lights are not turned on at one time. Cyclic lighting is useful for lighting several areas of the greenhouse in succession. At an intensity of 10 to 20 ft-c no more than 20% of a chosen cycle need be lighted. For example, if four hours is broken down into 30 minute cycles, the lights in a section would be on for six minutes (20% of 30 minutes) and off for 24. This would allow cycling among 5 stations and consume 80% less electricity. Control systems for cyclic lighting are more complicated.


Reducing the day length

      During the seasons of the year when natural photoperiods are longer than the critical photoperiod for a crop, short days can be simulated by covering plants with opaque material (light-proof cover) each evening and removing the material in the morning. Black cotton cloth (black sateen) or black plastic is widely used for this purpose. To be effective, the arrangement of supports above the plants and the black cloth itself must reduce the light intensity to below 2 to 3 ft-c. The standard weave for black cotton cloth is 64 × 104 threads per square inch.

      In the greenhouse, plants are covered with black cloth from 4 or 5 PM and uncovered at 7 to 9 AM which works well into the work schedule of employees. One problem in the south is heat buildup under the black cloth in the afternoon. Chrysanthemum for example will fail to flower if the temperature reach 90°F or above for more than a couple of hours while covered. In this case, the black cloth must be pull at 7 PM or later to prevent heat delay of flowering and then removed later in the morning. Black cloth may not trap heat as readily as black plastic. Some growers have devised methods of installing fans under the black cloth to remove heat.

      Some growers do not cover plants requiring short days on one of the weekend days, usually Sunday. Research has shown that this can be done. However, for each day that the plants are not covered, there is a delay of one day in blooming time. If more than one day per week is missed, there may be serious delay in blooming.

      Several greenhouse establishments have devised an automated system for pulling black cloth. This consists of a rigid leading edge for the cloth, continuous cables, a reversible electric motor, and limit switches. The cloth and continuous cables are attached to the rigid leading edge which is usually a length of pipe or conduit. The cloth may lie on the continuous cables or hang below the cables on shower curtain hooks or rings. Low-horsepower reversible motors with reduction gears and a shaft moves the cables attached to the rigid leading edge. Limit switches are used to stop the motor when the cloth reaches the proper position. Automated black cloth must be installed overhead and down the sides. One major advantage to an automated system is the savings in labor.

      One added advantage of an automatic black-cloth system is that it may also be used as a heat conservation system during the winter. Therefore, the system is used for photoperiod control in the summer and for heat conservation in the winter. For heat conservation, the cloth is pulled at dusk and opened early in the morning. Care must be taken, however on cold mornings to open the cover gradually to allow the cold air above the cloth to mix gradually with the warm air below. This slow opening prevents the plants from being suddenly chilled by cold air.

      Short-day treatment is continued only as long as it is needed. Generally, black cloth should be continued until flower buds are visible. However, in the case of chrysanthemums, black cloth should be continued until the flowers are showing color.

      Because of the effect of light on photoperiodic plants, growers should be aware of extraneous sources of light that may effect plants. It may only be possible to have plants under short days and night interrupted lighting in the same greenhouse by covering those requiring short days. Be aware off light from outside sources such as street lights or adjactent buildings. If there is any question, measure these light sources with a light meter.