Nabesei Co Ltd, an electronic parts maker in Japan, exhibited plants grown under irradiation of LED lights in three different colors at an exhibition that took place from April 15 to 17, 2009, in Tokyo.

In the experiment, plants of the same size were continuously irradiated with light from red LEDs, blue LEDs, and pink light from half red LEDs and half blue LEDs. Three weeks later, by comparing the plants with one another, it was discovered that the pink LED light most effectively promotes the growth of plants, the company said.

Instead of all wavelengths in the visible light range, plants absorb light with certain wavelengths to grow. For example, when they perform photosynthesis or come into bloom, red light around a wavelength of 660nm, which is the absorption peak for chlorophyll, promotes the growth. Meanwhile, when the plants form flower buds, blue light around a wavelength of 450nm promotes the growth, according to the company.

Comparing the plants under the three colors of light, those under the pink LED light had larger leaves and had generally grown in a more balanced way, while those under the red LED light grew slower than others and were smaller as a whole. And the plants under the blue LED light had fewer leaves and were spindly on the whole.

Depending on different plant types, the wavelength ranges that affect the growth of plants are also slightly different. Therefore, field tests to evaluate the irradiation time and other issues should be conducted at agricultural experiment stations from now on, Nabesei said.

As incandescent bulbs are to be eliminated in 2012 in Japan, the company plans to focus on the application of LEDs in illumination equipment for growing chrysanthemums. Red LED light can delay the formation of buds on chrysanthemums. Moreover, LED light keeps bugs away because the LED emission spectrum is deviated from the bugs' visibility curve.