As some of you remember, I used to run a website about chiles and the technology used to grow them. well it doesn't exist anymore and thought it would be wise to post some of the information I published there. today I'll post a brief article about light, plants and their relationship. please bear in mind the article was written a couple of years ago and some data might be outdated. will do my best to fix any such data. enjoy.
Part I: The Basics
Light & Plants
Light is one of the most basic requirements for the existence of plants. the energy source of plants is sugar. instead of consuming it like almost any other life form, plants create their food in a process called Photosynthesis. for this process to occur, the plants need water, carbon dioxide (CO2) and sunlight which is the source of energy stored in the sugar.
The Different Colors
The white light is composed of many different colors, just like demonstrated in a rainbow. different colors of sunlight can be seen in different parts of the world and even different times of day. plants react differently to various colors of light, and will actually use only some of them in Photosynthesis. the two main colors of light used by plants are red and blue. high concentrations of red light are manifested in yellow - orange light and blue in white - bright-blue light.
The color of light is labeled as color temperature. The color temperature of a light source is determined by comparing its chromaticity with a theoretical, heated black-body radiator. The temperature (in Kelvin) at which the heated black-body radiator matches the color of the light, is that color's temperature.
Part II: Hardcore Data
The light receiving photosynthetic pigments of the plant are located in the thylakoid membrane of the chloroplasts. the most common pigment in its quantity and is present in all green plants and algae is ‘Chlorophyll a’. other pigments are called secondary pigments and include ‘Chlorophyll b’ and carotenoids. to every photosynthetic pigment there is a typical absorption spectrum, meaning a different absorption ability of light in different wavelengths.
- Chlorophyll a – 420nm, 663nm
- Chlorophyll b – 453nm, 645nm
- Carotenoids – 420nm-480nm
The absorption by the Chlorophylls is mainly obvious in the red and blue-violet area, although absorption lines in other areas do exist. Chlorophylls a and b are distinguished by the wavelength at which maximum absorption occurs. the absorption lines of Chlorophyll a at both the blue and red ends of the spectrum are farther away than the absorption lines of Chlorophyll b. Chlorophyll quantities in a green leaf vary between 0.05%-0.2% of the fresh weight. the ratio between Chlorophyll a and Chlorophyll b is 2.5/1=a/b, although it is not constant due to the fact Chlorophyll a is more easily destroyed, meaning the quantitative ratio varies under different conditions.
Types of Plants
Sun and shade plants differ by various leaf qualities due to the amounts of light reaching them. shade plants allocate more resources to harvest light than CO2 fixation. This is why in shade plants the secondary pigments are produced in greater quantities – more Chlorophyll and less RuBisCO*.
* Ribulose-1,5-bisphosphate carboxylase (RuBisCO), is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as sucrose.
Part III: Artificial Light
Whether you grow indoors or simply need an additional light source, artificial lighting is a popular and relatively easy solution these days. today we are going to discuss some of the more popular solutions in artificial lighting, but first we will answer the question: “What makes a good light source?”
As you can read in Part I and Part II, plants use certain colors to convert the energy of light into a usable energy source (sugar). so the first thing a good light source should have is the main colors being used. obviously the perfect light source will have them all. another important quality a good light source should possess is high luminous intensity*, thus the amount of energy reaching the plant and eventually being converted is high. the last thing we need to remember when choosing a light source is cost. usually artificial lighting means the use of electricity as an energy source, as with standard light bulbs. electricity costs money, so a good light source will supply the right colors, will have high luminous intensity and of course will do all of that with as little electricity as possible. with the sun having all possible colors used by plants, extremely high luminous intensity, it’s no doubt the perfect light source and it can’t get any cheaper than free. it is perfect, but when that fails we need alternatives.
Incandescent lamps work on the Incandescence** principle and are possibly the most common type of light source used in private homes. contrary to what you might think, incandescent lamps are not a good light source. they only contain some of the colors needed by plants, mostly from the red end of the spectrum and the Lumen/Wattage efficiency is relatively low. meaning low luminous intensity and high cost.
Fluorescent lamps are a type of gas-discharge lamps and also a common light source. With a wider spectrum and much more flexibility in producing certain wavelengths, the fluorescent lamps are a much better light source than incandescent ones. the relatively high Lumen/Wattage efficiency will help keep the costs low with only few actual shortcomings. the first one being the still not high enough luminous intensity, meaning the lamp should be kept as close as possible to the plant. this might not be a real disadvantage with seedlings, but the cost is quite high with most mature plants. another one is the requirement of ballasts, which limits the possible lamp designs. compact versions (CFLs) with ballast built-in are sold, so one could just use a standard lamp holder, but the cost of those lamps is usually higher.
High Intensity Discharge Lamps
High intensity discharge (HID) lamps are arc based. The actual conductor varies depending on the color temperature and luminous intensity sought for. HID lamps are quite popular in the industrial sector, especially as an alternative light source for plants. with one of the highest Lumen/Wattage efficiency out there, they’re probably the best artificial light you can find. the most notable HID lamps in agriculture are probably Metal Halide (MH) and High Pressure Sodium (HPS), with the MH leaning toward the blue end of the spectrum and HPS to the red one. like the fluorescent lamps, they also require ballasts. some even require special conditions for them to operate. indeed, HIDs need much care in terms of investment, but they’re worth it.
Light Emitting Diodes
Light Emitting Diodes (LED) are semiconductor based solid state diodes. they are very common in electronics and as indicator lights. The current Lumen/Wattage efficiency is not enough to compete with some of the other solutions, but this recently hyped light source has its advantages. a possibility to replicate the wavelengths plants need is one of them. With the right components, it is possible to create an array with most possibly the best artificial spectrum. Another advantage is the lack of moving parts, something that influences the life span and usage for the better. the technology is still underdeveloped, but future developments should improve overall efficiency. for now it is not your best choice, even though it is a good one.
* Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction, based on the luminosity function, a standardized model of the sensitivity of the human eye. the SI*** unit of luminous intensity is the candela (cd), an SI base unit.
** Incandescence is the release of electromagnetic radiation, usually visible radiation, from a body due to its temperature.
*** International System of Units
I hope this helps.