INTERACTIVE IMPACT OF THE SPECTRAL COMPOSITION OF ADDITIONAL LIGHTING AND ROOT ZONE TEMPERATURE ON THE PRODUCTIVITY OF HYDROPONIC LETTUCE

Hydroponic lettuce production in greenhouses is becoming increasingly popular in Ukraine. However, there is a lack of research evaluating the impact of light spectra, root zone temperature, production systems, and lettuce varieties on yield. This study aims to determine the interactive effects of nighttime supplemental lighting, nutrient solution temperature, and production system on the growth and productivity of various lettuce varieties under suboptimal air temperatures typical for winter greenhouses. The study used three lighting regimes: no lighting (control), violet LEDs (spectrum R90:G0:B10), and warm white LEDs (spectrum R35:G42:B23). Two nutrient solution temperatures—12.6 °C (no heating) and 18.8 °C (with heating)—as well as two production systems—Nutrient Film Technique (NFT) and Constant Flood Table (CFT)—were applied for lettuce cultivation. Red and green lettuce varieties of four types were studied: leafy, heading, oakleaf, and romaine. The results showed that the absence of supplemental lighting and heating significantly reduced yield, while their combination positively influenced the dry mass of shoots. The highest productivity was achieved with the CFT system, especially when combined with violet LEDs, which had a higher red light component that stimulates plant growth. It was found that violet lighting significantly outperformed white LEDs, particularly when the nutrient solution was heated. The most sensitive lettuce varieties to root zone conditions and lighting were Adriana, Cedar, Red Sails, and Salvia. In particular, under violet LEDs, these varieties showed the highest productivity when the CFT system was used. This research suggests effective strategies for optimizing hydroponic lettuce cultivation in winter greenhouse conditions in Ukraine. The findings can be useful for farmers aiming to increase yield and product quality, as well as for optimizing costs under conditions of limited resources.