OrchidSafari ARCHIVES*

(Author's note: this article is the result of some minor arm-twisting by Lois Greer, precipitated by information exchanged on the OrchidSafari chats. The author is NOT a botanist or plant physiologist, and so apologizes in advance for any inadvertent misinformation.)

What is light?

"Light" is the spectrum of electromagnetic radiation that we can see. This whole spectrum includes radio waves, microwaves, infrared waves (heat), visible light waves, ultraviolet waves, X-ray, and gamma rays, with lots more in between. This list is in order of increasing energy, and decreasing wavelength--the higher the energy, the shorter the wavelength, and vice versa.

What is color?

What we humans "see" is the result of our eyes absorbing certain wavelengths of light; we call each wavelength a "color". It's much nicer to say, "What a lovely orange-yellow Phal", than to say "What a lovely 587 nanometer Phal"! But it would be the same thing; our eyes see each wavelength as a particular color.

What is pigment?

Sunlight (white light) is a mixture of all visible wavelengths, obvious when we split it with a prism or raindrops and see a rainbow: red, orange, yellow, green, blue, indigo and violet. Pigments (colored chemicals) are said to be "red" if they reflect red wavelengths of light; our eyes absorb it and we say "Red!". Since red is reflected alone, where did all the other light colors go? They were absorbed by the pigment chemical.

How does this affect my orchids?

Light is a form of energy. By basic physics (we'll leave out advanced nuclear physics here), energy cannot be destroyed or created, but it can be changed in form. Good thing, or we and our orchids wouldn't be here. Pigments in the leaves are primarily carotenoids and chlorophylls. Carotenoids are red, orange and yellow pigments that aren't very good at capturing light energy for other uses. Chlorophylls A and B are very good at energy capture but they absorb in the blue/violet and red regions only. That's why we SEE leaves as green--this is the reflected light, which is not absorbed.

This helps explain changes in leaf colors--the plant makes more chlorophyll and leaves are darker green if it needs to capture more energy. This is common in some genera if light is insufficient. With more light, the plant doesn't need to maintain so many copies of this large, expensive molecule--leaves to keep up their chlorophyll; colors we see are the usually-covered carotenoids.

Once the light energy is absorbed by the chlorophyll, it is used to power the formation of sugar molecules from water and carbon dioxide. These sugars are used for structure and energy for growth of leaves, roots and flowers. Photosynthesis is the term used to describe the energy capture and transfer process.

When teaching plant biology to my students, I have an unofficial list of "ways to kill your plants." High on the list is, "Give them green light!" Even at 10000 footcandles of intensity, green light would not be useful, because chlorophyll reflects it. Your light meter would say "OK" but your plant would die.

How does this correlate to lighting for orchids?

Growers knows that each species or hybrid, or even each plant, is an individual. But all use chlorophylls A and B, and all need red and/or blue light of the correct wavelengths to be happy. Sunlight provides all visible light energies, plus infrared (heat) and ultraviolet (UV, cause of sunburn). Also, it's rumored that orchids evolved using sunlight... When we use artificial light supplements, we need to provide the correct wavelengths (colors) to give the plant energy to grow and flower. Ordinary fluorescents do this; specialized plant lights are designed to emphasize the necessary red and blue wavelengths. Some of these impart an interesting red/violet glow--looking like you have something extraterrestrial happening in your plant room.

What about short-day and long-day plants?

Every flowering plant has some mechanism which controls when, and how extravagantly, it flowers. This varies from species to species-some will bloom whenever they have enough energy to do so; others base their blooming on temperatures. Some plants are said to be sensitive to day length. We call those that flower in summer "long day" plants and those that flower in winter "short day" plants. But we are wrong; they are really "short night" or "long night" plants.

What the plant "notices " is the length of the night, or uninterrupted dark period. This is due to a sensor pigment that changes when it receives light energy, then takes a while to be converted back to the "dark" form. Experiments show that we can fool a long day (really, a "short night") plant into blooming if we break up the night by a single flash of light. in fact, we could give the plant only a few hours of light per day, just enough energy to keep it functional, and coerce it into flowering by flashing a strobe every few hours during the dark cycles. The plant would respond as if it had experienced several short nights, not a broken long night, and it would bloom. Unfortunately, depriving the plant of optimum light energy for growth and development would take its toll, and the plant probably wouldn't last long! Similarly, a short day ("long night") plant may refuse to bloom if its dark period is interrupted by even minimal light. Thus, most recycled Poinsettias refuse to rebloom in household conditions.

A basic understanding of the properties of light and pigment are useful to ensure that your plants have the energy they need, and to understand their responses to varying light conditions.