Microclimate Effects On Ohio-Grown Baby Lettuce: Biomass Accumulation and Composition

Pre-harvest factors can limit the efficiency and productivity of vegetable cropping systems. Moisture, light, and root and shoot zone temperature influence plant biology and its manifestation in key traits, including yield (biomass, nutritional) and resource use efficiency and when abiotic conditions are sub-optimal, overall cropping system performance declines. Microclimate modification can reduce the occurrence of sub-optimal conditions. In this study, we expose lettuce to four aerial and root zone temperature treatments in two settings during spring and fall in Wooster, OH and document biomass accumulation and tissue composition. Plots are located within 0.61m x 2.44 m x 0.15 m raised beds filled with a four-component, soil-based organic medium containing mostly peat moss and compost. Half of all plots are situated outdoors while all other plots are located within a 9.1m x 24.4 m x 4.6 m high tunnel covered with a single layer of 6-mil clear film. Half of all plots in each setting are covered with 0.8-mil, clear, vented, low-tunnel film and half of all plots are underlain by electric heating cables triggered by temperatures below 23°C. The use of low tunnel film and cables provides the treatments uncovered-unheated, covered-unheated, uncovered-heated and covered-heated in each setting. Red-leaved romaine lettuce varieties (Outredgeous, Flagship) are direct-seeded (2700 seed/m²) in early October and late March, overhead-irrigated as needed and harvested multiple times over the next 28 d, concluding when plants meet ‘baby leaf’ market criteria. Light, relative humidity, soil and air temperature are monitored throughout the experiment which has been repeated twice (2 seasons/year x 2 year). Harvested tissue is frozen and laboratory measures of anthocyanin, chlorophyll, sugar and vitamin C content and total antioxidant capacity are taken. Root and shoot-zone conditions and variety have significantly affected leaf biomass and composition, typically in both seasons and settings. Leaf yield has been greater in low tunnel-covered and bottom-heated plots than in uncovered-unheated plots with effects most dramatic in the open field setting. Anthocyanin and chlorophyll levels and total antioxidant power have generally been highest in treatments displaying the lowest total yield, regardless of setting. The negative relationship between anthocyanin level and yield is particularly clear in open field plots. We take these data as further evidence of the potential to alter early-stage lettuce biomass accumulation and composition through root- and shoot-zone microclimate modification, particularly during fall and spring in the Midwest