Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2016
Publication Date: 1/1/2017
Citation: Lafta, A., Turini, T., Sandoya, G.V., Mou, B. 2017. Field evaluation of green and red leaf lettuce genotypes in the Imperial, San Joaquin, and Salinas Valleys of California for heat tolerance and extension of the growing seasons. HortScience. 52:40-48.

Interpretive Summary: Being a cool-season crop, lettuce is vulnerable to heat stress. Global warming poses serious threats and challenges to the production of leafy vegetables. To adapt to climate change, this study was conducted to evaluate the performance of leaf lettuce varieties for heat tolerance by growing them in different locations in California (Imperial, San Joaquin, and Salinas Valleys) that differ in temperatures during the growing season. Fifteen green leaf and twenty-one red leaf lettuce varieties were selected to evaluate their performance under these environments. These varieties were planted in March and May in Five Points and El Centro and in June, 2012 in Salinas. The results suggest that lettuce planting can be extended from January to March beyond the normal growing seasons in San Joaquin and Imperial Valleys, where yield may be higher than in the Salinas Valley. However, the further delay in planting date from March to May in Five Points and El Centro resulted in reduction of yield and an increase in susceptibility to bolting and stress related disorders such as tipburn and leaf desiccation in most varieties. The susceptibility to these disorders depends on the cultivar and the temperature during lettuce growth and maturation. Heat-tolerant leaf lettuce varieties adapted to these regions were identified. Results of this research should be useful for the development of heat-tolerant lettuce cultivars and extending the growing season in warmer but low land cost areas to reduce production costs, which help maintain or increase the profitability and sustainability of this important crop in a changing climate.

Technical Abstract: Global warming poses serious threats and challenges to the production of leafy vegetables. Being a cool-season crop, lettuce is vulnerable to heat-stress. To adapt to climate change, this study was conducted to evaluate the performance of leaf lettuce genotypes for heat tolerance by growing them in different locations in California (Imperial, San Joaquin, and Salinas Valleys) that differ in temperatures during the growing season. Fifteen green leaf and twenty-one red leaf lettuce genotypes were selected to evaluate their performance under these environments. These genotypes were planted in March and May in Five Points and El Centro and in June, 2012 in Salinas. The results suggest that lettuce planting can be extended from January to March beyond the normal growing seasons in San Joaquin and Imperial Valleys, where yield may be higher than in the Salinas Valley. However, the further delay in planting date from March to May in Five Points and El Centro resulted in reduction of yield and an increase in susceptibility to bolting and stress related disorders such as tipburn and leaf desiccation in most varieties. The susceptibility to these disorders depends on the cultivar and the temperature during lettuce growth and maturation. Heat-tolerant leaf lettuce varieties adapted to these regions were identified. Results of this research should be useful for the development of heat-tolerant lettuce cultivars and extending the growing season in warmer but low land cost areas to reduce production costs.