How many readers think that you need ultraviolet light to sustain photosynthesis in plants?
After all, whether they use sunlight or artificial illumination, greenhouses inevitably expose their crops to ultraviolet radiation.
A superb crop of basil after just a few weeks growth in the Vertical Farm. Not a single bug
(or any leaf damage!) to be seen . . .
Well, it turns out that UV is not necessary. To sustain photosynthesis and growth, plants only need red and blue light. They don’t need green light either; they reflect it, which is why plant leaves are green!
And they certainly don’t need ultraviolet; too much UV causes sun-burn to plants, just as it does with humans.
So it transpires that the artificial light needed for plants grown in greenhouses can be simply provided by high intensity red and blue LEDs. But why would you want artificial light anyway? Why not simply use the Sun?
Well Ol’ Sol is fine for traditional land farming but we’re talking about much more intense cropping – the factory farming of the future.
Actually, there has been a lot of research into the use of artificial light for photosynthesis.
In fact, plant biologists refer to photosynthetically active radiation (PAR) as light ranging in wavelengths from 400-700 nanometres, precisely what can be sourced from LEDs and since plants largely reflect green wavelengths, we are only interested in red and blue LEDs.
Here’s a complete Vertical Farm, taken with a crop growing and
the LEDs illuminated. The
containers in front hold the
nutrient solutions (with
their pumps); the
Photosynthetically active radiation (PAR) is usually measured in micromole photons per square meter per second (μmol photons/m2/sec); one micromole being one-millionth of a Mole.
A mole of photons is the same as Avogadro’s number: 6.02 x 1023. And it just happens that photosynthesis is directly proportional to the number of photons falling on the plants: about nine photosynthetically active mole photons is required to produce one mole of oxygen, the product of photosynthesis.
Incidentally, there is also quite a lot of current research into artificial photosynthesis, ie, where no plants are involved, just micro-organisms. For the purpose of this article though, we are interested in artificial illumination for use in green-houses.
We should also define the term “bioponics”. This is different from hydroponics in that the nutrient mix contains biologically active micro-organisms. The micro-organisms are crucial in enabling the plants to take up all the vital nutrients. Without the micro-organisms, hydroponics can be very much a hit-and-miss process.