(Translation of article from Dutch magazine "Onder Glas" September 2009)

For the first time the researchers of Wageningen university have succeeded to let plants grow in a climate chamber under artificial daylight. It’s a break through because now the influence of light colours on a plant can be researched much better. That’s important for instance for the LED research. Also the first results show that the sunlight spectrum makes the young cucumber plants 64% heavier than SON-T-light, at equal light strength. Reason to look at the colour of assimilation light.

Text; Sander Hogewoning, Wim Van Ipeperen & Tijs Tierkels. Images Wageningen University.

A climate chamber is used to investigate one or more factors while you have exact control of all the other factors. Only this way can you tell 100% for sure that it was the influence of the specific factor.

Trials in climate chambers have one enormous disadvantage; until recently it was not possible to provide daylight as the lamps to achieve this where simply not available. That made the results achieved in a confined space difficult to translate in practice. That counts strongly for instance at the research with LEDs. In the climate chamber applying LED light gives (in certain colours) often very steady results. If the researcher then repeats the trial in a glass house; he has next to the LED light also always daylight; with the matching spectrum. But then its often hard to translate the clear results from the lab in a usable implication for the glass house.

The Influence of Light on Morphology

A higher light intensity stimulates the photosynthesis and there by the production. here by always was less attention given to the effect that the spectrum of that light not only had influence on the photosynthesis, but also on the morphology (the shape of the plant). The colour of the offered light has effect on the stretching of the leaf and stems, the leaf stand, the opening of the stomata, and spouting of the flower bud and many more processes. This means automatically also that the bigger the light intensity that you hang up in artificial light the more of these colour dependable process you influence with your assimilation light.

Because the spectrum of those vary very much of that of the spectrum of the sunlight (see fig1) SON-T for instance has high peaks in greenyellow and orange light, but achieves bad in blue and red that strongly varies from the natural light.

In the climate chamber is now researched, financed by STW, PT and LNV, which influence that had. That was possible by using a plasma lamp (only recently available on the market) in combination with filters and additional lamps. With that the sunlight could be simulated very well (see fig 2) meanwhile the light intensity stayed under control. Because for a reliable comparison it has to keep the same of course.

The results are very clear. Young cucumber plants became 64% heavier (dry weight) under artificial sunlight then under SON-T-light and 128% heavier that under fluorescent light. That was not because of the photosynthesis per unit leaf surface, because this was exactly the same at a light strength of 100 µmol/m² , the applied light strength during the grow.

What was clearly different; the light capturing. That was very clear to see from the plants. With fluorescent and SON-T-light they stayed very flat; on the artificial sunlight the grow up normal. The sunlight provides – in comparison with fluorescent and SON-T-light – for longer stems, longer leaf stems and thinner leaves. The plant invests more in leave stretching than in leaf thickness. That tells that the surrounding area increases faster. And that means more photosynthesis. From the start these plants capturing more light. Thereby they assimilated more. There by you get even faster leaf separation, what leads to even more light capturing. It’s a profit on profit affect.

Thereby there are big differences at the young plants. At the start the plant clearly grows much faster through the morphological advantage, caused by the influence of the light colours.
In a fully grown vegetation this effect doesn’t accure anymore, because by then all the light is captured. Then the net-photosynthesis per unit leaf surface is important first at all and that is –like said- for the 3 treatments evenly.

The questions of course is now if it’s useful to hang up this type of artificial sunlight in the glasshouse instead of SON-T or LED’s (with which you can never offer a full sunlight spectrum). This questions can not be answered for this moment.

Especially by pre cultivate or by plant material cultivation it could be very important. And then especially in light poor periods, when the effect of artificial light as added lightning counts the hardest. But for now the most important advantage in this new technology lies in scientific research. The research for the influence of coloured LED’s as added lightning for instance can now take place more directed.

By the way this whole story does not say that results of earlier scientific test in climate chambers are questionable now. The understanding of many physiologic processes in the plant can develop itself without the daylight spectrum being offered. This stays that way. But in the previous years became more obvious that light colour has influence on the shape of the plant. That is now much better to research. Also the application of a artificial daylight spectrum can speed up the horticultural research enormously.

Researchers don’t have to do there tests in different seasons in their glasshouses to research at different day lengths and daylight intensities. The glasshouse situation for all seasons is at all time simulatable with an artificial daylight spectrum.

Recently a British/German company offers a Sulphur Plasma lamp of which the spectrum comes closer to sunlight then all the existing lamps. Three quarters sits in the for the human eye visuable light between 400 and 700 nanometer. This lamp produces almost no ultraviolet light and just a little infrared. The basics of the lamp are a bulb Sulphur (with some other chemicals) that is heated up to a glowing plasma with micro radiation. The technology was already developed in the 90’s by NASA and the American ministry of defense, but knew long time no commercial application.
The website www.plasma-i.com gives a clear view of the technical status. The system is presented on the Hortifair in 2008.

To be clear: the spectrum of the lamp was corrected with filters and extra lamps (by the research team in the Wageningen universities department of horticulture) to come as close as possible to actual sunlight.

In tests under artificial sunlight in a climate controlled room it shows that young cucumber plants grow much better then under SON-T or TL-light. That’s because the colour of the light has influence on the shape of the plant. At the right light colour the young plant is much more capable of capturing light. Tests with artificial daylight are only possible since recently. They are in great interest at the research with coloured LED’s and can speed up horticultural research.