Red to far-red ratio correction in plant growth chambers

Unlike natural light, growth chamber light sources typically have a high ratio of red to far-red (R:FR) light, hence incandescent lamps are commonly added to the light mix to reduce R:FR. However, R:FR still remains well above the natural ratio of 1 in the proportions commonly used in growth chambers, which for some species may still relatively delay flowering and reduce length. Incandescent lamps also produce a lot of heat, which in itself can be a major influence on plant growth, and are also being phased out in many countries in favour of more energy efficient sources. This study examined the influence of R:FR correction in growth chambers by incandescent lamps or FR light emitting diodes (LEDs). Spectral scans to 1100 nm show the significant heat loading of incandescent relative to LEDs. Growth studies using 4 species (pea, tomato, barley and arabidopsis) showed little influence of ratio correction by either method on barley and tomato, while pea and arabidopsis were only responsive to large reductions in R:FR. Temperature and humidity measurements both increased at plant level with the use of incandescent, but not with LEDs. This had little influence on the growth of barley and tomato, but did influence pea and arabidopsis. The influence was less in barriered light loft growth chambers. The results overall suggest that use of incandescent lamps has little influence in the usual proportions provided in growth chambers, while higher proportions can increase temperature and humidity at plant level. Much lower R:FR is achievable through the use of FR LEDs without such influences.