Supplemental lighting with LED for efficient year-round production of soilless tomato in a Mediterranean greenhouse

Supplemental lighting (SL) is currently arising increasing interest in Mediterranean greenhouse for efficient year-round production. Compared to traditional high-pressure sodium lamps, light-emitting diode (LED) grow lights provide important advantages [a generally touted higher efficiency in terms of micromoles of photosynthetic photon flux (PPF) produced per unit of electricity; a provided light with a spectral distribution used efficiently by plants; the possibility to provide specific spectra able to induce specific responses by plants]. In this research we compared the effects of inter-light LEDs ['R+B': red (660 nm) + blue (450 nm) + red (660 nm) light; 'FR': red blue + blue red + infrared (730 nm) light] toward natural light growing conditions ('C', control) on the performance of soilless greenhouse tomato ('DRW 7723' and 'Diamantino' cultivars, both grafted on 'Maxifort'). The experiment was carried out at the experimental farm “La Noria” (Mola di Bari, South Italy). SL was operated automatically by a system composed by a CR1000 datalogger and a quantum sensor: for 16 h d-1, the system evaluated every 4 s the incoming sunlight in the greenhouse, and turned on the LED light bars whenever the ambient PPFD dropped below 200 μmol m-2 s-1 until the PPFD from sunlight rose above the set-point again. The duration of the supplemental light activation ranged from about 7 to 16 h d-1 (values registered in October 2018 and November-December-January 2019, respectively) depending on the ambient light conditions during the growing cycle. Model equations describing daily light integral (DLI) trends inside the greenhouse and power consumption as affected by light conditions were obtained. Plants grown without SL showed, on average, 19% lower yield and 16% lower fruits number than tomato plants grown with SL. Also, water use efficiency and fruit quality traits were positively affected, but the response can be highly dependent on the cultivar.