Substantial nighttime Evapotranspiration in lettuce grown under controlled-environment conditions

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Abstract
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 water flux and water-use efficiency (WUE). 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. ET was partitioned into evaporation (E) and transpiration (T) components using the fraction of canopy cover derived from digital image analysis. During early development, E was reduced by 87% in covered modules. After canopy closure (23–28 days after sowing), the nighttime-to-daytime ET ratio was 0.46 for the uncovered treatment and 0.42 for the covered treatment (p < 0.001), indicating reduced nocturnal water loss. Nighttime stomatal conductance remained high, averaging 84% (536 daytime; 450 nighttime mmol m−2 s−1) of daytime values in the uncovered treatment and 77% (615 daytime; 476 nighttime mmol m−2 s−1) in the covered treatment. Silicone-rubber covers reduced cumulative water use by 32% without affecting biomass production, increasing WUE from 2.5 to 3.8 g L−1. The higher WUE in the covered treatment resulted from both E suppression and reduced nighttime water flux. These results demonstrate that substantial ETₙ persisted after canopy closure due to sustained stomatal opening, while surface covering improved WUE by reducing both E and nighttime water loss.

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