Water Use Efficiency of Four Types of Lettuce (Lactuca sativa) in Response to Different Drought Severities

Lettuce is the most consumed vegetable crop, according to a 2008 USDA product summary. Climate change may increase the frequency and severity of droughts, which can adversely affect lettuce production. The economic importance of this crop and its sensitivity to drought create the need to understand the response of lettuce to drought stress. The goal of this study was to understand growth and water use efficiency of lettuce under drought. Four types of lettuce (Bibb, Romaine, Iceberg and Loose leaf) were grown in a glass-covered greenhouse under a range of irrigation treatments to induce different drought stress conditions. To achieve different irrigation volumes and drought severities, five different drip emitters (2, 4, 6, 8, and 10 L/h) were used on each irrigation line. Each type of lettuce had a soil moisture probe in a pot with a 10 L/h emitter. When the volumetric water content in this pot dropped below 0.40 m³·m^-3, an irrigation valve was opened for 10 s, irrigating all plants of that particular cultivar. The use of different drip emitters assured the different plants received different amounts of water. Chlorophyll index increased with increasing drought severity in all four types of lettuce. No significant difference was found in leaf chlorophyll fluorescence among the different lettuce types. Total leaf area, total leaf number, and shoot dry weight decreased significantly with increasing drought severities in all types, except for Iceberg. All four types showed an increase in water use efficiency under increasing drought severity, with Loose Leaf lettuce being the most responsive. Our results show that water use efficiency in lettuce increases under drought stress conditions, which is important to consider when growing lettuce in areas with limited water availability.