Initial Evaluation of Lettuce Collections for Drought Tolerance

Lettuce (Lactuca sativa L.) production in California represents a 1.5 billion dollar accounting for nearly 80% of all lettuce produced in the United States.  While California has an abundance of fertile farmland and a favorable climate for the production of leafy vegetables, a combination of factors pose a serious threat to the future of the lettuce industry.  Limited water availability for irrigation arising from reduced precipitation along with regulatory reductions is already applying pressure on producers, while climate change threatens to exacerbate these effects.  Leafy vegetables with high water content, such as lettuce, are particularly vulnerable to drought stress.  A reduction in water applied could represent not only a substantial savings to the producer, but it could also represent the potential for an increase in production acres in order to keep up with rising demand due to population growth.  In order to identify lettuce cultivars adapted to limited water availability, more than 1,500 lettuce varieties and germplasm were screened in greenhouses for drought-tolerance through a binary drought screen evaluating leaf relative water content, wilt-resistance, and shoot fresh weight.  Selected varieties and germplasm, identified through initial greenhouse analysis, were further evaluated in the field under a gradient of water-stress conditions to identify cultivars showing resistance to limited water availability.  With a combination of greenhouse and field evaluations, differences in the ability of lettuce genotypes to adapt to limited-water environments were identified.  This initial screening of lettuce germplasm represents a substantial first step in developing cultivars improved for production in regions with increasing pressure on freshwater availability along with providing another tool by which producers and breeders can choose cultivars adapted for limited-water stress.