Influence of Supraoptimal Air Temperature and Cultivar on Yield, Morphology, and Production Considerations of Green Leaf Lettuce in Greenhouses

Lettuce ( Lactuca sativa ) is an economically important leafy green widely grown in greenhouses, yet there are limited data on the interaction between lettuce cultivar and air temperature for many cultivars currently marketed for commercial greenhouse production. Even in climate-controlled greenhouses, internal air temperature can exceed general recommendations, leading to bolting, excessive stem elongation, bitter flavors, and reduced yields. This study aimed to generate benchmark yield and morphological data for 20 lettuce cultivars grown hydroponically in a greenhouse during a fall (20 °C mean air temperature) and summer (28 °C mean air temperature) production cycle, with harvests at 9 (juvenile stage) and 21 (mature stage) days after transplanting (DAT). For both fall and summer, lettuce grew in a common nutrient film technique system with an average pH and EC of 5.6 and 1.5 dS·m −1 , respectively, with greenhouse air temperature setpoints of 21 °C (day) and 18 °C (night), and a target average daily light integral of 17 mol·m −2 ·d −1 . The experiment was set up as a randomized complete block design with two blocks. Depending on the cultivar, air temperature, and harvest time, lettuce yield and top projected canopy area (TPCA) were significantly different. However, regardless of the cultivar or harvest time, yield (kg·m −2 ·year −1 ) was almost always greater at 20 °C (fall) than 28 °C (summer). Supraoptimal air temperatures increased TPCA at both 9 and 21 DAT. Supraoptimal air temperatures decreased specific leaf area, resulting in thicker leaf lamina. Chlorophyll concentration was more affected by cultivar than harvest date or air temperature, but prolonged supraoptimal air temperatures decreased chlorophyll concentration at 21 DAT. Benchmarking yield and morphology across cultivars and seasons provides a tool for growers to improve crop selection and production strategies, while informing breeding efforts for improved controlled environment performance with regard to plant architecture and leaf morphology for greenhouses using automated harvesting and packaging.