Supplemental LED lighting in high-wire system cultivation in cucumber: what is the benefit of inter-lighting on production?

High-wire training system in cucumber production widely spreads in European commercial greenhouses. Optimization of plant architecture benefits to high plantation density and thus improves yield, reduces waste, and facilitates pest management although a significant increase of labor is needed. Mitigation of cost balance can be performed by a modification of production calendar to profit to highest selling prices using supplemental lighting. Because of large leaf area, basal plant part is prone to shading in high-wire cucumber training and inter-lighting is widely used among French producers. This lighting placed within the crop canopy takes the advantageous of low bulb temperature of light emitting diodes (LEDs) technology while significantly increased lighting cost. Its impact on crop production is still in debate because of season/natural light conditions, and installed light diversity, quantity, and quality in literature. To decipher advantages provided by LED inter-lighting, two years of experiment with two five-month crops each were conducted inside a 312 m² heated glasshouse. Fruits were harvested three times a week. Two full LED lighting systems were installed to compare different vertical light distributions: Top-lighting (TL) system provided a light intensity of 210 µmol m^-2 s^‑1 from the top of the canopy, while top + inter-lighting (T+IL) system provided a light intensity of 135 µmol m^-2 s^‑1 at the top of the canopy and 75 µmol m^-2 s^‑1 within the canopy. For two winter seasons, yield and harvest fruit number were not significantly different between installed systems demonstrating that inter-lighting is not required for high production performance although electric consumption was reduced by 16%. During summers, inter-lighting increase light use efficiency and economic profitability over TL. An increase of gross margin of 4.5 € m^‑2 was achieved using only inter-lighting without affecting natural photoperiod. Lower electric consumption, and ability to control both light levels independently favored the use of inter-lighting solution in commercial high-wire cucumber production.