LED Inter-Lighting in Year-Round Greenhouse Mini-Cucumber Production

Inter-lighting, applying part of supplemental light within crop canopy, can improve light distribution within canopy and thus increase crop yield and light use efficiency. Inter-lighting with HPS (high pressure sodium) lamps has been successfully tested in Scandinavia. However, the high bulb temperature with HPS has prevented its use in the popular twin-row greenhouse vegetable cultivation system (with small row width). Light emitting diodes (LEDs) have low bulb temperature, making it a potentially suitable light system for inter-lighting. The 'waste' heat generated by the inter-light LED may also be beneficial to the crop as our previous study with grow pipes has shown that adding some heat inside crop canopy improves microclimate and cucumber productivity. Light quality can affect the content of antioxidants and produce quality, and a specific spectral composition may trigger a special crop response. Therefore, we investigated the response of greenhouse mini-cucumber to LED inter-lighting with different spectra. Mini-cucumber plants were grown in 2 greenhouses during the winter. Top-lighting (145 mu mol PAR m(-2) s(-1)) was provided with HPS lamps. Control (no LED inter-lighting) and 3 LED inter-lighting treatments (red, blue and white LEDs, 14.5 mu mol PAR m(-2) s(-1), 10% of the top-lighting) were applied inside both greenhouses. All LED inter-lighting treatments improved fruit visual quality. They also increased fruit yield beyond the increase in PAR (more than 10%) in early production period. However, the yield increase with LED inter-lighting gradually diminished toward the late production period. The smaller crop canopy and lower canopy coverage over the LED inter-lighting system in the late growing season might have reduced inter-light interception, decreasing its beneficial effects. Further winter experiments are being conducted to improve mini-cucumber response to LED inter-lighting in late growth period.