Principles of Light

By Erik Runkle, PhD

(Courtesy of the  American Orchid Society)

 High-pressure sodium lamps are the most common lamp type
 used in greenhouses because they are the most efficient 
 at converting electricity into photosynthetic light. 
 photo: © Erik Runkle, PhD.

Light seems like such a simple phenomenon, yet it is deceivingly complex, especially with regard to its effects on plant growth and development. Light can influence essentially all aspects of orchid plant growth and flowering. First and foremost, it provides the energy for photosynthesis, or the conversion of carbon dioxide and water into sugars and oxygen. This article describes the three dimensions of light and the relevance of each to growing orchids.

LIGHT DURATION
The duration of light in a 24-hour period is known as the daylength or photoperiod. Photo-period changes throughout the year, and the magnitude of the change depends on the latitude. In equatorial regions, the photoperiod changes less than 30 or 40 minutes from winter to summer. As one moves farther away from the equator, the change in photoperiod during the year increases. For example, the natural photoperiod (the duration from when plants first perceive light before sunrise until when light is no longer perceived after sunset) in northern Florida (30°N latitude) ranges from approximately 11 hours in late December to 14.5 hours in late June. In northern Oregon, southern Minnesota and central New York (45°N latitude), the photoperiod varies from 9½ hours to just over 16 hours. These photoperiods are about 30 to 40 minutes longer than the period from sunrise to sunset, because there is enough light for plants to perceive for 15 to 20 minutes before sunrise and after sunset.

Photoperiod is one of the few consistent environmental parameters every year. Thus, it is no surprise that some plants use the change in photoperiod as a signal to induce flowering at a favorable time of the year. Numerous floriculture crops flower only when the day is sufficiently short (or more accurately, when the night period is sufficiently long). A few common examples are poinsettias and chrysanthemums. Some orchids also flower in response to short days, including some species and hybrids of Cattleya, Dendrobium and Phalaenopsis. However, there is relatively little research-based information on the effects of photoperiod on flowering of most orchid genera and their hybrids. The photoperiod can be readily manipulated by hobbyists and commercial growers. When the photoperiod is naturally long (from March to September in temperate regions of North America), short days can be created by covering plants with an opaque black cloth or plastic. When plants are not exposed to natural daylight, lamps can be set to be on for less than 12 hours per day. A word of warning: plants can perceive low levels of light — even sometimes one tenth of a foot-candle, which isn’t enough for most people to read a book. Therefore, beware of light from adjacent rooms polluting your orchids if a short-day photoperiod is desired.

Phalaenopsis grown under excessively low light
levels develop thin, narrow leaves.
photo: © Matthew Blanchard.

The photoperiod can more easily be extended by providing artificial lighting. To create a long day, a small amount of light (10 foot-candles or more) is required. This intensity can be delivered by one 60-watt incandescent or one 15-watt compact fluorescent lamp every 5 to 6 feet (1.5 to 1.8 m) apart. This amount is not enough to sustain plant growth, but is enough to create a long photoperiod if that is desired.

What photoperiod should you provide your orchids? There is not one correct answer, as it depends on what types of orchids are in your collections. For example, if some of your plants routinely flower during the winter when the natural day length is short, then you could inhibit flowering if you provided a long day. Growers primarily interested in increasing growth during nonflowering times may want to provide a long day (e.g., 16 hours of light). Continuous lighting can cause undesirable responses in some plants, so at least a six-hour dark period is recommended each day.

LIGHT QUALITY
Light consists of individual particles of energy called photons. Each photon within the visible light spectrum has the potential to drive photosynthesis. Light quality refers to the spectral distribution of light, or the relative number of photons of blue, green, red, far-red and other portions of the light spectrum that is emitted from a light source. Some of these portions are visible, whereas others are not. The energy of each photon is dependent on its wavelength. Photons with a short wavelength, such as that of ultra-violet (UV), have more energy than photons with a longer wavelength, such as red light.

The wavelength of light is most commonly measured in nanometers (nm), which is one billionth of a meter. Blue light is generally considered to be the portion of light that has a wavelength between 400 and 500 nm. Blue light, green light (500 to 600 nm) and red light (600 to 700 nm) compose the spectrum of light that is primarily used for photosynthesis (400 to 700 nm). Approximately half of the energy that comes from the sun falls within the photosynthetic waveband. The remaining amount of energy has shorter wavelengths (such as UV light) or longer wavelengths (such as infra-red radiation).

Leaf scorch on phalaenopsis, which can occur when
a direct beam of light is greater than about
1,500 foot-candles.
photo: © Matthew Blanchard.

The proportion of red light relative to the amount of far-red light (the red to far-red ratio) influences stem elongation, particularly in orchids that grow under high light. Lamps that emit large amounts of far-red relative to red light (such as incandescent lamps) promote plant elongation. In addition, plants are effective filters of red light, but transmit or reflect most far-red light. Therefore, in an environment shaded by plants (such as under a forest canopy), the red to far-red ratio decreases and extension growth of crops below is promoted.

Sophisticated devices called spectroradiometers, which measure light quality, are expensive ($4,000 or more). However, greenhouse growers and hobbyists usually don’t need to measure light quality because it is relatively fixed for each light source. The distribution of light for the most common light sources is summarized in Table 1. The radiant yield refers to the percentage of energy that is in the form of photosynthetic light. High-pressure sodium lamps for supplemental green-house lighting are used widely primarily because they have the highest radiant yield.