Substantial nighttime evapotranspiration in lettuce grown under controlled-environment conditions

Nighttime evapotranspiration (ETₙ) has been widely reported across plant species, yet its magnitude and physiological significance in controlled-environment crop production remain poorly understood. This study quantified ETₙ in lettuce (Lactuca sativa L. cv. Outredgeous) grown under controlled-environment conditions and evaluated nighttime-to-daytime ET dynamics under contrasting surface cover conditions. Plants were grown in peat-based root modules operated as zero-drainage systems, allowing ET to be quantified from continuous mass loss of individual nutrient reservoirs. Two surface treatments were compared: uncovered growth media and growth media covered with flexible silicone-rubber sheets incorporated as part of the evaluated growth-system configuration, which also suppressed evaporation (E). ET was partitioned into modeled E and modeled transpiration (T) components using the fraction of canopy cover derived from digital image analysis. During early development, modeled E was reduced by 87% in covered modules, enabling evaluation of ET n under minimized E conditions. After canopy closure, when T accounted for nearly all measured ET, the hourly nighttime-to-daytime ET ratio (ET n /ET d ) averaged 0.46 for the uncovered treatment and 0.42 for the covered treatment (p < 0.001), indicating that substantial nighttime water loss persisted primarily through T in both treatments. However, these ET n /ET d ratios were only approximately half of the estimated apparent nighttime-to-daytime canopy stomatal conductance ratios (gₛₙ/g sd ), which averaged 0.82 and 0.75 in the uncovered and covered treatments, respectively. This difference indicates that nighttime ET was limited mainly by reduced evaporative demand, even though apparent canopy g s remained relatively close to daytime levels. Overall, silicone-rubber covers reduced cumulative water use by 32% without affecting biomass production, increasing water-use efficiency (WUE) from 2.5 to 3.8 g dry matter L -1 nutrient solution. These results demonstrate that substantial ETₙ persisted after canopy closure and was consistent with sustained nocturnal stomatal conductance under controlled-environment conditions, indicating that ET n can represent a major non-productive component of crop water use when E is minimized. Accounting for ET n may therefore improve ET estimates, water-use assessments, and environmental control strategies in controlled-environment agricultural systems.