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Alternatives of agricultural chemicals such as growth retardant is awaited from the environmental point of view. Blue light would have potential to shorten plant height, and growth analysis may make effective timing and period of application of blue light clear. And it will be one of the basic data to construct a quantitative model to describe the relation between internode and/or stem elongation and light condition. This technique would provide growers with an environmental friendly ways to regulate plant growth of ornamental crops.

Experiments were conducted in a controlled environment cabinet that equipped temperature control, eight controllable fluorescent lamps, blue LEDs. A turn table was used to increase efficiency of experiments. A CCD camera which had sensitivity in infrared region and an infrared illuminator were employed to obtain an image of plants during day and night.. The plant images were obtained every 10 minutes over three weeks.

Two light conditions were planned for the experiments, that is to say, Exp I :12hrs light(0600 to 1800), 12 hrs dark(1800 to 0600) with 4 hrs night interrupt(2200 to 0200) by a fluorescent lamp, and Exp.II : 12hrs light(0600 to 1800), 12 hrs dark(1800 to 0600) with 4 hrs night interrupt (2200 to 0200) by blue LEDs (0400 to 0600). It was observed that the growth speed of internode length was slow under in the light period, and it became fast in the dark, and that the average value of the amount of internode elongation per day was reduced even to about 60% by blue LED, and that the inhibitory effect of internode elongation by blue LED was maintained not only in the night interruption period irradiating blue LED but in the dark and light periods following it.