Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

Anker Kuehn, AgroTech,, • Eva Rosenqvist, KU-Life, • Stig Gejl, Philips Lighting,

Artificial lighting and

lighting control of the future

Lighting diodes are becoming increasingly efficient. In a few years time they

are expected to be even more efficient than SON-T lights, and prices will have

dropped so much that they can compete with other types of lighting.

It should also be mentioned that LED lights have a longer life-time

So there are many advantages to using

lighting diodes. The question is, are

there other advantages to using LED

lights? One of the advantages is that you

can change the colour composition and

thereby affect the growth of plants. For

example, colour composition affects the

way a plant stretches and when it flowers.

Effectiveness of photosynthesis

Chlorophyll is the most important pigment

and its function is to absorb light in plants’

photosynthesis. It absorbs most light in the

red and blue area. Plants are green because

they reflect much of the green light and

absorb other colours in the light spectrum.

In principle, a plant can use light of any

colour in its photosynthesis because light

is absorbed by different pigments, and

energy from the light is then transferred to

the photosynthesis reaction centre.

Blue light contains more energy than

red light, although both colour wavelengths

contribute equally to photosynthesis.

In terms of energy, the same amount of

Watts will render more red light rays than

blue. That is, as regards photosynthesis, it

is “cheaper” to use red light than blue.


The flowering pattern of many plants is

governed by the available light. Some

plants flower when there is enough light,

that is when there is a photosynthesis

“surplus”, while the flowering pattern of

other plants is regulated by the length of

daylight. In plants that are regulated by

daylight, the length of night is the most

important. Plants contain a substance called

photochrome that can appear in two

forms, active and inactive, and it is this

substance that determines whether the

plant will flower. Photochrome can be

converted between its active and inactive

forms by using light in the red and farred

region. We know that we can bring

poinsettias to flower a week early if we

draw the curtains before the sun sets.

This is because the amount of light from

the far-red region that normally shines on

the plants just before the onset of night is


Shape and height

There is much more far-red light than red

light in a woodland area (the leaves in the

tree crowns are very efficient at absorbing

red light), and this acts as a signal to the

photochrome in the leaves below, leading

these plants to stretch toward the light. We

also see this in plants that are exposed to

incandescent lamps, as these also irradiate

a lot of far-red light.

In addition to this phenomenon, blue

light also affects the shape of plants. Plants

that are exposed to more blue light are

more compact and produce more side

shoots. This is due to the similar conditions

to plants growing freely under a blue sky.

Correspondingly, plants that are exposed

to UV light thus stretch less.

Artificial lighting can

be controlled but it is only

supplementary lighting

When we know how plants react, we

can control the light and the colours of

the light, to match the plants’ needs. This

means that we do not need the same light

composition all of the time; sometimes

only extra photosynthesis is needed, and

in such situations it would be cheaper to

use red light, while at other times we need

to affect the shape of the plant and therefore

other light colours are needed.

The question is, how do we make this

work in practice, given that most of the

current knowledge has been obtained from

climate chambers and experiments in the

lab? In the Innovation Consortium project,

Greenhouse Concept 2017, we are testing

the LED technology in the greenhouse.

However, there are lots of light combi-

nation possibilities, and lots of different

plant reactions, so we will probably see

many different LED combinations, possibly

in the same lamp cabinet.

We must remember that daylight is

the most important light source and that

artificial light is merely supplementary

light. We know from SON-T lights that

yellow-orange light is a useful supplement

to daylight, however it does not work

very well on its own because it results in

undesirable stretching. When we use the

lights as supplementary lighting, they must

be well suited for photosynthesis, but we

can achieve even more by using new light

colour combinations. And the expected

increase in efficiency and the longer lifetime

of the LED lights is added bonus.


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