2019 ASHS Annual Conference

Hydroponic growers typically add supplemental lighting (SL) and heating in greenhouses to grow crops during winter. While studies indicated that spectral composition of SL provided during the day has little effect on crop growth, little research explored differences in spectral composition of SL provided at night. Moreover, comparative studies between conventional methods of heating greenhouse air versus using heated hydroponic solution with cooler greenhouse temperature (HHS) are limited. In addition, the benefit of using HHS and nighttime SL in the nutrient film technique (NFT) and deep flow technique (DFT) systems and for different lettuce cultivars is not known. The objective of this study was to evaluate the interactive effect of nighttime SL with different spectral composition and HHS in NFT and DFT systems on the growth of different lettuce varieties in a greenhouse maintained at suboptimal air temperature. The treatments comprised of three light treatments [no SL (4.2 mol·m^-2·d^-1), red/blue SL (13 mol·m^-2·d^-1), and white SL (12.6 mol·m^-2·d^-1)], each with two solution temperature treatments [unheated (12.6 °C) and heated (18.8 °C)] and further each temperature treatment containing two production systems (NFT and DFT). Red and green cultivars belonging to leaf, romaine, butterhead and oak leaf groups were grown in each production system. The greenhouse air temperature was 15.6/10 °C (day/night). Results indicated a significant light× temperature× production system interaction for shoot fresh and dry weights. Lettuce growth was higher under DFT than NFT and the difference was more pronounced in the heated than unheated treatment in both SL treatments, with larger differences under red/blue LED and heated compared to white LED and heated treatments. However, there were no differences in fresh and dry weights between NFT and DFT in both temperature treatments when SL was not added. In addition, a significant light× production system× variety interaction was observed for shoot fresh and dry weights. There were no differences among varieties in both production systems under white LED and no SL treatments. Whereas, growth of three varieties (Cedar, Red Sails, and Salvius) was significantly higher under DFT than NFT when exposed to red/blue LED supplemental lighting. Based on these results, highest shoot fresh and dry weight of lettuce varieties was observed under red/blue SL given at night with HHS. Moreover, fast growing varieties like Cedar, Red Sails, and Salvius are preferred under red/blue SL and DFT production system.