16 June 2015

Lights for your plants

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Plants are an essential part of our lives, from the provision of food to aesthetics. Just think about how much is spent on Valentine ’s Day - Americans spend US$1.9 billion on just flowers! One of our team members bought the Click And Grow system with great expectations. Unfortunately, the results were less than stellar. A quick search showed that many variants of such domestic self-contained “plant growth systems” exist - each with their own strengths.

We know that light is essential for plant growth. Sunlight is free and a natural source of light available, however it is not always attainable in sufficient quantity for domestic horticulture. Therefore, the use of artificial light has become very common to increase production and quality.

Generally less known is that plants absorb only certain spectrums of light. Plants do not absorb all incoming sunlight (due to respiration and reflection of light, etc.) and do not convert all harvested energy into biomass. Natural daylight has a high color temperature (approximately 5000-5800K) but only 45% of the light is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11%.

The specific wavelengths relevant to photosynthetic absorbance have been detailed in many sources of literature. In selecting a spectrum, one must understand various conditions such as photoperiodicity, luminous efficacy and photosynthetic efficiencies specific to the plant being cultivated.

What this means is that plants absorb only certain wavelengths of light and, instead of using broad-spectrum lights, the use of specific-spectrum LEDs is gaining wider adoption in horticulture-agriculture to increase plant yield and improve energy efficiencies. Plant growth lamps are designed to stimulate growth by emitting EM spectrums correct for photosynthesis. Growth lamps are very applicable in areas where or when sunlight is scarce, such as during winter months, when the available hours of daylight are insufficient for desired plant development, and also to extend the amount of time that plants receive light.

Using specific wavelengths of light is an energy-saving approach by avoiding light in spectrums unusable for plant growth. Also, when compared to other types of lights, LEDs do not require bulky ballasts and produce less heat than incandescent lamps. Because of this reduction in heat, plants transpire less and thus the time between watering cycles is longer – saving both water and energy!

To first understand what spectrum of light plants need, we need to know that there are three types of photosynthetic pigments that absorb light, and two types of chlorophyll in photosynthesis. Red-light-sensitive pigments are known as phytochromes and blue-light sensitive pigments are known as cryptochromes.

Without going too in depth into the specific science of photosynthesis and respective photosystems, generally speaking, the two most important absorption peaks of chlorophyll are located in the visible regions from 625 to 675nm (red) and from 425 to 475nm (blue).

Chlorophyll absorption peaks are 430nm and 662nm for chlorophyll A, and 453nm and 642nm for chlorophyll B. Chlorophyll B is not as abundant as Chlorophyll A, but helps to expand the wavelengths of light absorption. In a simpler sense, there is more Chlorophyll A (which absorbs more red light) and some Chlorophyll B (which absorbs more blue light). There are also other forms of chlorophylls (C1, C2 , D and F) found only in the photosynthetic members of the Chromista, cyanobacteria and  dinoflagellates and not in typical plants and they will not be discussed here.

 

 

So we can now clearly see several wavelengths of absorption for plants, and these spectrums are measured as photosynthetic photon flux (PPF)/PAR. There are multiple absorption peaks for chlorophyll and carotenoids, and LED grow-lights may use one or more LED colors overlapping these peaks. PAR (Photosynthetically Active Radiation) light is used to measure light useful for plants, whereas the brightness of a light that is measured in lumens is useful only for humans.

Vegetables tend to prefer blue (~400nm). However, for growing fruits or flowers, a greater proportion of red is preferable (600-640nm). As with exposure to continuous far-red light, blue light can also promote flowering through the mediation of cryptochromes photoreceptors. Moreover, blue-light-sensitive photoreceptors - flavins and carotenoids (such as carotene and xanthophyll) are also sensitive to the near-ultraviolet radiation-sensitivity peak at around 370nm.

To truly match the absorption peaks of Chlorophyll A and B, you will need four specific wavelengths, 430nm, 453nm, 642nm and 662nm. Unfortunately, such specific wavelengths are hard to come by and thus it would seem logical to choose lights that peak in the 430-470nm and 640-680nm range. To allow the two main chlorophyll types to gather the most energy. Noting that there is more Chlorophyll A than B, a greater ratio of red lights to blue should be utilized, about 3:2 (red:blue).

With this knowledge, we cobbled together a quick prototype just to hold an off-the-shelf growth lamp and a 3500K warm lamp for aesthetic effect in hope of salvaging our unhealthy plants. Future variants will have humidity/temperature sensors as well as ambient lighting/environmental control sensors for more optimal growing conditions.

Check it out!

Finished prototype

Close up of the two lamps

 

For fun, we installed remote controls to the lamps, pretty neat.

If you like this article and would like to build your own plant growth systems, do drop us a message!

Read 2366 times Last modified on 28 June 2015

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