【正文】 late 1980s, LED designers used similar techniques to produce highbrightness and high reliability LEDs. This led to the development of InGaAlP (indium gallium aluminum phosphide) visible light LEDs. Via adjusting the energy band gap InGaAlP material can have different color output. Thus, green, yellow, orange and red LEDs could all be produced using the same basic technology. Also, light output degradation of InGaAlP material is significantly improved. Shuji Nakamura at Nichia Chemical Industries of Japan introduced blue LEDs in 1993. Blue LEDs have always been difficult to manufacture because of their high photon energies ( eV) and relatively low eye sensitivity. Also, the technology to fabricate these LEDs is very different and less advanced than standard LED materials. But blue is one of the primary colors (the other two being red and green). Properly bining the red, green, and blue light is essential to produce white and fullcolor. This process requires sophisticated software and hardware design to implement. In addition, the brightness level is low and the overall light output of each RGB die being used degrades at a different rate resulting in an eventual color unbalance. The blue LEDs available today consist of GaN (gallium nitride) and SiC (silicon carbide) construction. The blue LED that bees available in production quantities has result 4 in an entire generation of new applications that include telemunications products, automotive applications, traffic control devices, and fullcolor message boards. Even LED TVs can soon bee mercially available. Compare to incandescent light’ s 1000h and fluorescent light’ s 8000h life span, LEDs have a very significantly longer life of 100,000 h. In addition to their long life, LEDs have many advantages over conventional light source. These advantages include small size, specific wavelength, low thermal output, adjustable light intensity and quality, as well as high photoelectric conversion efficiency. Such advantages make LEDs perfect for supporting plant growth in controlled environment such as plant tissue culture room and growth chamber. Table 1 is a list of some mon types of LEDs as piled from . Table 1 Some mon types of LEDs. Peak wavelength(nm) Color Material and structure of LEDs Substrate 730 Farred GaAs GaP 700 Red GaP:ZnO GaP 660 Red GaAs 650 Red GaAs 630 Orange– red :N GaP 610 Orange :N GaP 590 Yellow :N GaP 585 Yellow :N GaAs 565 Green GaP:N GaP 450 Blue GaN/SiC – 3. Color ratios and photosynthesis The chlorophyll molecules in plants initiate photosynthesis bycapturing light energy and converting it into chemical energy to help transforming water and carbon dioxide into the primary nutrient for living beings. The generalized equation for the photosynthetic process is given as: CO2 + H2O— light— （ CH2O） + O2 where (CH2O) is the chemical energy building block for thesynthesis of plant ponents. Chlorophyll molecules absorb blue and red wavelengths most efficiently. The green and yellow wavelengths are reflected or transmitted and thus are not as important in the photosynthetic process. That means limit the amount of color given to the plants and still have them grow as well as with white light. So, there is no need to devote energy to green light when energy costs are a concern, which is usually the case in space travel. The LEDs enable researchers to eliminate other wavelengths found within normal white light, thus reducing the amount of energy required to power the plant growth lamps. The plants grow normally and taste the same as those raised in white light. 5 Red and blue light best drive photosynthetic metabolism. These light qualities are particularly efficient in improving the developmental characteristics associated with autotrophic growth habits. Nevertheless, photosynthetically inefficient light qualities also convey important environmental information to a developing plant. For example, farred light reverses the effect of phytochromes, leading to changes in gene expression, plant architecture, and reproductive responses. In addition, photoperiod (the adjustment of light and dark periods) and light quality (the adjustment of red, blue and farred light ratio) also have decisive impacts on photomorphogenesis. The superimposed pattern of luminescence spectrum of blue LED (450– 470 nm) and that of red LED (650– 665 nm) corresponds well to light absorption spectrum of carotenoids and chlorophyll. Various plant cultivation experiments are possible when these twokinds of LED are used with the addition of farred radiation (730– 735 nm) as the light source. Along the line of the LED technology advancement, LEDs bee a prominent light source for intensive plant culture systems and photobiological researches. The cultivation experiments which use such light sources are being increasingly active. Plant physiology and plant cultivation researches using LEDs started to peak in 1990s and bee inevitable in the new millennium. Those researches have confirmed that LEDs are suitable for cultivation of a variety of algae, crop, flower, fruit, and vegetable. Some of the pioneering researches are reviewed in the followings. Bula et al. have shown that growing lettuce with red LEDs in bination with blue tubular fluorescent lamp (TFL) is possible. Hoenecke et al. have verified the necessity of blue photons for lettuce seedlings production by using red LEDs with blue TFL. As the price of both blue and red LEDs have dropped and the brightness increased significantly, the research findings have been able to be applied in mercial production. As reported by Agence France Press, Cosmo Plant Co., in Fukuroi, Japan has developed a red LEDbased growth process that uses only 60% of electricity than a fluorescent lighting based one. Tennessen et al. have