Variety-Specific Lettuce Responses to Oxygen-Enriched Water and LED Light in a Controlled Greenhouse: A Multivariate Analysis Across Two Cycles

Investigating the impact of oxygen-enriched water combined with LED light on lettuce growth for two consecutive cycles is essential for advancing greenhouse cultivation in Mediterranean climates, where summer heat poses significant challenges. This study investigates the combined impact of oxygen-enriched water (O₂EW) and LED lighting on physiological, biochemical, and growth responses of two lettuce cultivars across two cultivation cycles in a controlled environment. The two lettuce types, Lactuca sativa var. ‘Lollo Bionda’ (Lugano) and Lactuca sativa var. ‘Lollo Rosso’ (Carmesi), were cultivated employing the Nutrient Film Technique (NFT) method within a regulated greenhouse setting. A randomized complete block design (RCBD) evaluated lettuce growth in an NFT system under three treatments: natural water (NW), oxygen-enriched water (O₂EW), and O₂EW with LED light (380–840 nm) (LED + O₂EW). The plants were exposed to natural oxygen levels (NW) of 6.2–7.4 mg L⁻¹ in the first and 7.4–8.1 mg L⁻¹ in the second period. Under O₂EW, levels reached 8.0–8.6 mg L⁻¹ and 8.7–9.2 mg L⁻¹, respectively, while LED + O₂EW concentrations were 8.4–8.5 mg L⁻¹ in the first and 8.8–8.4 mg L⁻¹ in the second period. The PPFD for ‘Lugano’ ranged from 426 to 486 µmol m⁻² s⁻¹ in the first cycle and 437–511 µmol m⁻² s⁻¹ in the second, averaging 448.66 and 460.65 µmol m⁻² s⁻¹, respectively. For ‘Carmesi’, it ranged from 421 to 468 and 441 to 492.3 µmol m⁻² s⁻¹, with averages of 438.66 and 457.1 µmol m⁻² s⁻¹. Statistical analysis was performed using two-way ANOVA and Tukey’s HSD test (p < 0.05) in IBM SPSS Statistics (version 29.0.2.0). The applied treatments significantly influenced the plants’ physiological parameters, including the photosynthetic rate, stomatal conductance, transpiration rate, and antioxidant activity. These treatments also significantly (p < 0.05) affected plant growth metrics such as the height, diameter, mass, number of leaves, root length, root mass, as well as biochemical components like chlorophyll, nitrate, and glucose content. The applied treatments significantly enhanced plant growth, biochemical components, and physiological parameters. Via comparative analysis, we concluded that the overall physiological performance of the plants in the second cycle was approximately 21.18% higher compared to the first cycle when combining all attributes. ‘Lugano’ showed stronger growth in height, mass, and root traits, while ‘Carmesi’ excelled in antioxidant activity, especially under LED + O₂EW treatment. Oxygen treatments boosted photosynthesis and transpiration in both varieties, with ‘Carmesi’ showing higher rates and ‘Lugano’ demonstrating greater growth, especially in the second cycle. In conclusion, O₂EW and LED treatments significantly enhance lettuce growth and resilience, particularly under warmer conditions, highlighting their potential to support sustainable year-round greenhouse cultivation.