Finding the optimal growth-light spectrum for greenhouse crops

Abstract: Especially in an open crop (e.g., young plants) morphological responses to light quality can affect light interception, crop photosynthesis and growth. Earlier work showed a substantial morphology related biomass increase for young cucumber plants grown under 100% artificial sunlight (ASL) compared with 100% high pressure sodium light (HPS). Here, ASL is used to investigate the effect of HPS and LEDs compared with ASL, when applied supplemental to an ASL background. Tomato plants were grown in a climate room under 17 h ASL (50% of in total 200 µmol PAR m-2 s-1) supplemented with 50% HPS, light emitting diodes LEDs (red/blue), or ASL. The 100% ASL-grown plants produced 32-45% more dry weight, due to a more efficient light interception. As ASL lamps are not energy-efficient enough for commercial production we tried to simplify the solar spectrum while retaining enhanced crop productivity in greenhouses. Red/blue/far-red LEDs, at a ratio inducing the same phytochrome photostationary state (PSS) as natural sunlight, and sulphur-plasma lamps, emitting a continuous spectrum in the PAR-region, were tested and compared with supplemental red/blue LEDs, HPS and ASL in a greenhouse experiment. Red/blue/far-red LEDs resulted in a visual appearance similar to the ASL-plants, while red/blue LEDs produced the most compact morphology. Red/blue/far-red LEDs enhanced dry weight for cucumber (+21%) and tomato (+15%) compared with HPS. Dry weight and compactness were intermediate for sulphur-plasma. The differences were attributable to effects of leaf orientation and positioning on light interception, and not to photosynthesis per unit leaf area. The PSS appears to be a key-factor to control crop morphology, providing a tool to induce ‘sunlight’ crop characteristics to enhance productivity.