Effects of Red and Blue Light Emitting Diodes (LEDs) on the Growth and Development of Lettuce (var. Youmaicai)

Lettuce (Lactuca sativa var. Youmaicai) was cultivated in white light emitting diode (LED), red LED only and combination of red and blue LEDs. The relevant growing characteristics of lettuce plants were studied. The results showed that red light could promote the radial growth of lettuce plants, and blue light was propitious to their transverse extension and synthesis of photosynthesis pigments. Lettuce plants couldn't grow normally in red light only. There were some positive effects of blue light (5–20%) on the growing of lettuce plants, for instance, the improvement of photosynthesis, transpiration and fluorescence (Fo, Fv/Fm and qN), the increase of contents of photosynthesis pigments and nutrients, the enhancement of total biomass. The most growing parameters that blue LED proportion was in the range of 10% to 20% were higher than those in white LED. The negative effects of blue LED on the growing of lettuce plants appeared with the increment of blue light proportion (25–50%). The biomass of the shoots decreased significantly, which induced the increasing ratio of root/shoot. The best lighting condition for the growing of lettuce was the combination of 90% red and 10% blue light.