Effect of Blue Light and Temperature on Leaf Expansion, Stem Elongation, and Growth

Short height and high yield per unit energy in controlled environments are essential to the success of a food production system for spaceflight. Temperature and light quality can be manipulated in controlled environments to reduce plant height and increase yield. Although the effects of temperature on height and yield are well studied at ambient CO2, temperature effects at elevated CO2 with a hydroponic root zone are not well characterized. We studied soybean yield and height under two lamp types over a broad range of temperatures. Temperature had little effect on yield or height, but lamp type had a significant effect on canopy height. This first study highlighted the importance of understanding spectral quality in controlling plant growth, especially canopy height. Numerous studies have compared lamp types and suggested that profound differences in leaf area, canopy height, yield, and total dry mass responses were due to blue light differences. Unfortunately, the most energy-efficient light sources have the least blue light. We have a poor understanding of the specific morphological and histological effects of blue light on leaves and stems. Three species, soybeans, wheat, and lettuce, were grown at five blue light fractions (0, 2, 6, 12, and 26%) and two light levels (200 and 500 μmol m-2 s-1). Phytochrome photoequilibria were constant among treatments. Blue light responses were species dependent. Wheat leaf area, dry mass, and stem length were insensitive to blue light fraction. Increasing blue light to 26% decreased soybean stem length, but leaf area was greatest at 6% blue. Lettuce leaf area, stem length, and dry mass were highly sensitive to blue light fraction between 0% and 6% under high pressure sodium lamps, but were insensitive between 6% and 26% under metal halide lamps. These results may be complicated by sensitivity to other wavelengths. The decrease in soybean stem length with increasing blue light was caused by an inhibition of cell division, while the decrease in leaf area was caused primarily by a decrease in cell expansion. Increased lettuce leaf area with increasing blue light fraction was caused by both cell division and expansion. This research indicates that lamps high in blue photons are not only energetically wasteful, but do not benefit, and in some cases reduce, plant growth. However, some blue light is necessary for controlling plant height in soybean and even required for proper growth and development in lettuce