Sustainable Lettuce: Adaptability to Uncertain Production Conditions

Lettuce is a popular and widely consumed leafy vegetable.  California and Arizona annually produce more than 250,000 acres of iceberg, romaine, leaf, and specialty types of lettuce, supplying more than 95% of the U.S market as well as exports to Canada and other countries.  These states have dominated lettuce production in the U.S. since the 1920s.  Lettuce is best suited for production in temperatures that range from 18 to 25°C, and production rotates with the seasons to production districts in California and Arizona where this temperature range predominates.  Cultivars that are adapted to different soil conditions, photoperiods, and plant stresses are needed to insure a year round supply of lettuce.  There are increasing concerns that climate change may impact the sustainability of this production system.  Unexpected periods of higher than ideal temperatures may increase the occurrence of premature bolting and thermoinhibition, as well as the physiological defects tipburn and rib necrosis.  Nearly all the water needed to grow lettuce in the western U.S. is provided through irrigation, and there are concerns about the future availability of water for crop production.  The geographical range of plant pathogens and pests may change, leading to the regular occurrence of biotic stresses currently considered rare or exotic.  The effect of climate change on pest and pathogen severity, and the resulting amount of economic loss, will depend upon the specific interactions between the host, the pest or disease, and the production environment.  Lettuce is a diploid, autogamous crop with extensive genetic variation within the crop and in its wild relatives for resistances to biotic and abiotic stresses.  Breeding approaches suitable for inbred crops have been used successfully to develop improved cultivars, and can be applied to develop cultivars adapted to the stresses anticipated to occur in the future.  Other unanticipated stresses may nonetheless occur, and new breeding techniques and genetic resources may need to be adopted or developed in lettuce to accelerate the breeding of new cultivars.  This will shorten the time between identification of new production constraints and release of improved cultivars.