Monday, April 15, 2013

Light for Plants

The absorption of chlorophyll a and b in a solvent. source
Botany is complex. Not every chemical reaction in photosynthesis is understood. What we have learned about plants and light has evolved the standards and design of high-tech agriculture lighting. This is a summary of the colors plants absorb and how they respond to them. (I will be happy to elaborate about any subject mentioned here that needs more explanation in the comments!)

Sunlight appears more red in autumn and winter as the sun rises lower on the horizon. Sunlight must pass through more of earth's atmosphere during these seasons and blue light is reflected away. This is also why sunsets are red or gold colored. Many plants are sensitive to an increase in red light as a signal to begin flowering before winter. In spring, as the sun rises higher in the sky, more blue light penetrates the atmosphere allowing the spring surge of vegetation.

There are 3 groups of plants based on their responses to light: Long Day Plants, Short-Day Plants and Day-Neutral plants. In each of these categories it is the duration of darkness(not light) that stimulates a response from plants.
  • Long-day plants flower in the spring as days are getting longer and sunlight is more blue. Lettuce, peas, turnips, wheat, clover, and carnations are varieties of long-day plants.
  • Short-day plants blossom in the autumn or late-autumn in the northern hemisphere as the duration of darkness increases and sunlight is increasingly red(after June 21). These plants require uninterrupted periods of darkness and do not flower if their night-time periods are interrupted by several minutes of light. These include plants like coffee, chrysanthemums, strawberries, corn, cotton, hemp, rice and sugar cane.
  • Day-neutral plants respond to some other stimulus than light to initiate stages of development. They may respond to changes in temperature, nutrient availability or achievements in developmental maturity. These include plants like cucumbers, roses and tomatoes.
For short-day plants, increasing exposure to red light will provoke flowering just as a change in season(summer to autumn) would. Long-day and neutral plants don't need an increase in red light to flower and will continue to mature under blue light. By changing the duration, spectrum and intensity of exposure with indoor lighting, botanists can control the stages of vegetation, flowering and fruiting.

Plants use many pigments to absorb and react to light. Pigments are divided by their purpose into three categories: phototropins, cryptochromes and phytochromes.

Phototropins are pigments that allow plants to respond to light by affecting the curvature of growth, the triggering of stomatal(pores) opening or developmental changes. Phototropins are the reason plants bend towards light and react in many other ways to light exposure.

Cryptochromes absorb light in the blue spectrum specifically at 380nm and 450nm (pterin, flavin). These pigments mediate phototropism, circadian rhythms and gene expression. Blue light promotes stem elongation, and leaf expansion. Cryptochromes are targeted in the vegetation stage of indoor agriculture with metal halide grow bulbs or other blue-light bulbs.


Phytochromes absorb red(650-670nm) and far-red(705-740nm) light. The color of these pigments alternate in response to the absorption of light. (Exposure to red light changes the phytochrome to preferentially absorb far-red light, while far-red light changes the phytochrome back to absorb red light again.) Gene signalling and expression are driven in the far-red stage of absorption. Without both red and far-red light, plants will become developmentally stunted. Light bulbs like high pressure sodium provide light in the red and far-red spectrum to stimulate short-day plant maturation and flowering.


The apparent(visible) colors of plants are complementary to the colors of light they absorb. The complementary color of absorbed light is the actual color of the plant pigment(s).

Pigments as complements to absorbed light.
absorbed λ absorbed color complementary color
These colors are based on peak values expressed in hexadecimal color.

Indoor Lighting for Agriculture
High Intensity Discharge(HID) light systems use bulbs like metal halide and high-pressure sodium. These systems have an effective light distribution and are the most widely-used for indoor agriculture. They also use more energy and release more heat than alternative agriculture lighting. HID's require a Socket, Ballast & Bulb.

Hoods and reflectors are fixtures that protect and insulate bulbs. Many are designed to be fitted with ventilation ducting to remove heat. Most hoods contain sockets that will take both MH and HPS bulbs from 250 to 1000 watts.

Digital Ballasts  |  Magnetic Ballasts
Ballasts convert the energy supply to a frequency that will light an HID Bulb. Some magnetic/analog ballasts are designed specifically for MH or HPS bulbs, while digital ballasts accept both. Conversion bulbs are made to be cross-compatible with ballasts designed specifically for MH or HPS bulbs. The wattage of the bulb and ballast must match.

MH Bulbs  |  HPS Bulbs
Metal Halide (MH) bulbs provide more blue light for the Grow, or vegetative stage that begins a plant's life cycle.
High Pressure Sodium (HPS) bulbs provide more red light for the Bloom, or flowering stage at the end of a plant's life cycle.
Please consider all of your energy usage in amps to grow safely with indoor bulbs. Read more: Power Capacity

Alternative Lighting

Fluorescent lighting includes T5 high output and compact fluorescent lighting. These are common because they produce little heat, require less energy and produce reasonably high light output. Compared to HID lighting, plants do not grow as tall beneath the lower intensity of fluorescent lighting.

Light Emmiting Diodes(LEDs) are low-energy, low-heat, long-lived color-specific bulbs used most often in electronics. They are now used in indoor agriculture as a color-specific supplement to stronger light sources. LEDs strong enough to grow plants are less efficient and generate more heat than flourescents with the same output.

The most common function of lighting controllers is to automate the day-night cycle of an indoor garden. Basic timers are inexpensive and worth the money to avoid manually regulating a grow cycle. Specific controllers have many uses, like delaying the power when a bulb is switched on to prevent situations like a hot-start. Multi-system controllers may regulate temperature, CO2 and humidity in addition to high-intensity lighting. These controllers have multiple programmable outlets, but are ultimately limited by the circuit capacity.

New Lighting Technology
Science continues to provide new solutions for indoor agriculture lighting. While new lighting technology is usually more expensive, the cost is offset by lower energy requirements and longer-lasting bulbs. We keep an eye on new energy-efficient lighting systems and their availability in agriculture.

*This content is published with modification on our main website:

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