Tipburn is one of the physiological disorders associated with heat stress and the symptom appears as brown necrosis around the margin of the affected leaves. These physiological disorders were influenced by genotype, environment, and the interaction between the genotype and the environment ( Jenni and Yan, 2009). Production of lettuce at higher temperature ranges resulted in losses of yield and quality and led to some physiological disorders such as rib discoloration, tipburn, premature bolting, and ribbiness ( Jenni, 2005 Jenni and Yan, 2009 Ryder, 1999). Lettuce ( Lactuca sativa L.) is a cool-season crop it achieves optimum growth and development at an average temperature of 18 ☌. Keywords: Lactuca sativa yield tipburn bolting heat stress leaf injury stress tolerance Results of this research should be useful for the development of heat-tolerant lettuce cultivars and for extending the growing season in warmer but lower land cost areas to reduce production costs. However, heat-tolerant leaf lettuce genotypes adapted to these regions were identified. The susceptibility to these disorders depends on the genotype and the temperature during lettuce growth and maturation.
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 heat-related disorders such as tipburn and leaf desiccation in most genotypes. 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. These genotypes were planted in March and May in Five Points (San Joaquin Valley) and El Centro (Imperial Valley) and in June 2012 in Salinas (Salinas Valley). Fifteen green leaf and 21 red leaf lettuce genotypes were selected to evaluate their performance under these environments. 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 within California that differ in temperatures during the growing season. Being a cool-season crop, lettuce is particularly vulnerable to heat stress.
Global warming poses serious threats and challenges to the production of leafy vegetables.
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