Artificial Lighting and Photosynthesis
14th Apr 1999
There's no doubt whatsoever in my mind that the use of artificial lights has improved immensely both the size and quality of our exhibition leeks and onions, indeed without them I have no doubt that the current world record weight of 15lb 15½ ounces for the heaviest onion would never have been achieved by Mel Ednie.
Why do plants therefore respond to this added light source that makes them grow at a much faster rate than they would normally? The answer of course is quite complex as the whole subject of lights and plant growth is an intricate one but I do want to try and explain that adequate lighting is important if you are not only to achieve better growth and end results, but also not to waste money on what might be a totally inadequate system. Let me try therefore and explain in my own way how light, both natural and artificial, can improve our leek and onion plants.
Spectrum and Radiation
Light is a word that is generally used to describe the portion of the sun's radiation which is visible to the human eye and is only a small part of its total radiation output which is called the Electromagnetic Spectrum. This Spectrum also includes ultraviolet radiation which gives us sunburn and infrared radiation which keeps us warm as well as X rays, Microwaves and Radio waves. The part of the radiation that concerns plant growth is the ultraviolet, visible and near-infrared regions which are described as electromagnetic waves and the distance between each wave is called the wavelength.
In this part of the sun's spectrum the wavelengths are so small, they are only a thousand millionth of a metre long and are measured in nanometres abbreviated as nm. When I served my time to qualify as a Toolmaker I used to work to what I thought were extremely fine tolerances, being much less than a thousandth of an inch, I can therefore just about grasp how small these wavelength really are.
Hope you are still with me! Our eyes can detect and see visible radiation between 400 and 700 nm and our bodies can also detect infrared radiation, though invisible to our eyes we can feel the warmth. The seven colours of the rainbow appear between 400 and 700nm and the ones we are most sensitive to and see the best are greens and yellows which fall between 500 and 600nm. Visible light as seen by the human eye is measured in Lux which is defined to include the wavelengths as seen by our eyes; this is the traditional way of measuring light as it represents the sensitivity of the human eye. Because we use visible light to see our way through the world, our eyes are accustomed to both very bright and very low light levels.
Photosynthetically Active Radiation
Plants also detect and use the above radiation levels between 400 and 700nm but with a much wider sensitivity than is seen by the human eye. The above range is important too for the photosynthesis process which produces sugars, hence growth and is called the Photosynthetically Active Radiation or PAR region, which is the light level for plants and is measured in units of Watts per square metre. Plants are more sensitive to the Red colour regions, then blue and least of all to green colours, in fact they reflect most of the green wavelengths which is why they appear green to us. Remember in school the letters ROYGBIV?, they are the 7 letters that always enable me to remember the rainbow's spectrum order of colour and decreasing wavelength; Richard Of York Goes Battling In Vain, i.e. Red, Orange, Yellow, Green, Blue, Indigo, Violet. It is therefore important to try and understand that plants and humans view the sun's radiation in very different ways and remember, plants are not able to utilise the very low light levels which humans can easily see by.
Good Quality Light
For an efficient photosynthesis process to take place, the quality of light is just as important as the amount of light received by the plant. 'Good quality' light for plant growth contains the correct ratios of the different colours and wavelengths between 400 and 700nm, such as appears on a bright sunny Summer afternoon. If the light level is poor when a young leek or onion plant is starting to grow away, it can grow tall and spindly as it tries to grow fast to ‘reach' the light. To our eyes, it seems that there is simply a low light level, but the plant is actually seeing shortage of the red wavelengths in the light.
In contrast if there is a surplus of the red wavelengths compared to other colours, the plants will grow short and bushy, in most cases this will result in a healthy looking plant. To measure the quality of light a plant is receiving, it is usual to measure the ratio between the red wavelength at 660nm and the far-red wavelength which is just outside the PAR range at 730nm. On a bright, sunny Summer afternoon, the red to far-red ratio equals one. At low sun angles such as early morning and early evening, the red/far-red ratio is often less than One showing that even natural sunlight, believe it or not, is often not ideal for good plant growth throughout the day.
To conclude this, it is important to measure light quality and the red/far-red ratio when giving your leeks and onions supplementary lighting. Different lamp types give different colour ratio lighting which can dramatically effect the red/far-red ratio. For example, quartz halogen lamps give out more blue wavelengths while tungsten lamps give out more red wavelengths.
This therefore must mean that giving plants a source of light from just one or two tungsten bulbs or one or two fluorescent tubes, though the light appears plentiful to you through the human eye, it is probably very inadequate for the complete and optimum growth of the plant and a waste of your money. Sadly these meters are expensive and cost over three hundred pounds so I shall continue to monitor the light levels in my growing cabinet by using the traditional Lux meter that I purchased many years ago.
I checked the Lux reading in my growing cabinet at the end of February on a bleak rainy day, the cabinet is 12 ft long and 3ft wide with 2 Philips SGR light units at 30 inches above the bench level with 400 watts SON T Aggro lamps in each unit. When the meter was placed directly on the bench and directly underneath the lamp I had a reading of 3,000 Lux. When the meter was moved 1 metre along the centre of the bench the reading was even higher at 3,300 Lux which shows how good this type of lamp is at evenly dispersing the light along the bench. At this sort of light level my onion and leek plants have been the best ever this year and I would suggest that this is the minimum light level to aim for which is really not in comparison to even an overcast Summer's day as the table below shows.
Interesting figures for typical daylight levels with readings in Lux.
Natural light Artificial light
Clear night, full moon 0.3 lux Candle at 1m distance 1 lux
Winter's day, overcast sky 900-2,000 lux Side roads 4 lux
Summer's day, overcast sky 4,000-20,000 lux Main roads 16 lux
Winter's day, clear sky up to 9,000 lux Staircases 30-60 lux
Summer's day, clear sky up to 100,000 lux Hallways 120 lux
Living rooms, offices 250 lux
Classrooms, shops workshops 500
Drawing offices, precision workshops 1,000 lux
If you are still awake and interested in more information on the above, Light Measurement Guidance Notes are available from - Skye Instruments Ltd, Unit 32 Ddole Industrial Estate, Llandrindod Wells, Powys, Mid Wales LD1 6DF