Part I: Background

Light, just like water, soil, temperature and air, is indispensable for plant growth. Man had realized the importance of water, soil and temperature in growing plants since a long time ago, and had begun to utilize irrigation, fertilization, and greenhouse technologies to artificially alter the conditions and environment for the plants. As a result, the yield was successfully improved. However, we’ve always had little knowledge of lighting and its influence on plant growth.
Man used to believe that plants could obtain all the necessary nutrition from the soil until the year 1880, when American scientist G. Engelmann found through experiments that green leaves could produce starch and release oxygen only with light. It was only after then that the lighting condition appeared in plant research and cultivation.
Normally the human eye can respond to wavelengths from 380nm to 780nm, (deep violet to near-infrared). Similar to the human eye, plants can absorb wavelengths from 400nm to 700nm, which is defined as photosynthetically active radiation (PAR). That is to say, only lights (wavelengths) from 400nm to 700nm can be perceived and absorbed for photosynthesis by plants.  
 
Studies find that light influence plant growth in three aspects:

1. Light intensity.

It is known that plants can absorb CO₂, produce chemical energy, and release O₂ only with light. In darkness, they respire, takes in O₂, consume chemical energy, and release CO₂. When the light intensity is 0, they only photosynthesize, and the higher the light intensity is, the faster the photosynthesis occurs. When photosynthesis and respiration happen at the same rate, the CO₂ generated from latter is entirely absorbed by the former, and the light intensity at this point is defined as the light compensation point. Only when the light intensity is higher than the light compensation point can plants grow healthily. The rate of photosynthesis grows as light intensity does, but it stops when light intensity reaches a certain point, which is defined as the light saturation point.

Figure 1. Relationship between illumination intensity and photosynthetic rate
 

2. Photoperiodism

Photoperiodism is the physiological reaction of plants to the length of day and night. According to their requirements for length of light and darkness in the 24-hour time cycle, plants are categorized as short day plants, long day plants and day-neutral plants. Photoperiodism is formed during the plant’s long-term adaption to the earth environment. Studies find that decreasing or increasing the lighting period can stimulate or postpone blooming.  

Figure 2. Effect of dark interruption on flowering
(Source: http://knowledgeclass.blogspot.com/2015/01/photoperiodism.html)
 

3. Light quality.

Visible white light is a mixture of lights of various wavelengths. The spectrum is different when the mixing ratio is different, and creates different visual experiences in human eyes. Color temperature is used to differentiate white lights, for instance the color temperature at noon is about 6500K, and it is normally around 2700K at dawn and dusk. For white lights of the same color temperature, the human eye evaluates their quality by the color rendering performance, which is highly depended on the spectrum. Plants in different stages of growth need lights of different spectrums. That’s why there hasn’t been a universal index just like CRI for describing and evaluating light quality. As we can see, plant growth has strict requirements for lighting. That’s why it is unpractical to grow low value crops, technically and economically.  Before LED technology was developed enough, traditional artificial lights played a limited role mainly as supplementary lighting in greenhouses. Now as LED technology burgeons, more plant factories are using LEDs as dominating light source, and they even appear in highly populated cities.
 
So compared to traditional artificial light source, what are the advantages of LEDs that bring the revolutionary changes to plant factories? We’ll talk about it in our next post of this plant growth series. Please STAY TUNED.