2018 ASHS Annual Conference

Salinity is a concern in the major lettuce growing districts in California and will be increasingly important. Lettuce is sensitive to salt stress, which reduces biomass and causes other undesirable effects. We sought to identify physiological traits, proteins and genes important in salt tolerance that can be selected for and introduced in new cultivars in order to adapt to high salinity without affecting productivity. Following up on information from previous salinity tolerance studies, crisphead lettuce cvs. Laura and Early Bird were chosen for more detailed analyses. The cultivars were evaluated in a growth chamber at 20°C, 50% relative humidity, 250 µmol/m²/s continuous light and Hoagland nutrient solution without or with NaCl and CaCl₂. Salt concentration in the salt treatment started from 0 mM/0 mM NaCl/ CaCl₂ (2100 µS/cm) at the time of seeding and was gradually increased every week to 30 mM/15 mM NaCl/ CaCl₂ (8600 µS/cm) to prevent salt shock. These high salt concentrations are non-lethal, permissive to lettuce growth and representative of salt concentrations commonly observed in areas of high salinity. We measured leaf chlorophyll content, photosynthetic CO₂ assimilation, leaf transmittance, photosynthetic efficiency (Fv/Fm) and fresh weight at the end of the 4-wk growth period. Shoot and root tissues were harvested for detailed analyses, including quantitative proteomics. Plants in the growth chamber produced significantly more shoot biomass compared to previous salinity-tolerance studies conducted in the greenhouse, and more clearly expressed phenotypic differences between control and salt treatments. Salt-sensitive ‘Laura’ had a much larger decrease in shoot mass compared with the salt-tolerant ‘Early Bird’. Salinity significantly increased chlorophyll content and photosynthetic CO₂ assimilation, but did not affect Fv/Fm in sensitive and tolerant lines. This contrasts with observations in other crop and model plants in which salt stress significantly decreases chlorophyll, photosynthesis, and Fv/Fm. The physiological responses of ‘Laura’ and ‘Early Bird’ to salinity are interesting for further studies that could identify unique adaptations in lettuce that may be important in salt tolerance. 1472 proteins from ‘Laura’ and 975 from ‘Early Bird’ were identified using quantitative isobaric peptide labeling, off-gel fractionation, and LC/MS/MS analysis. Twenty-eight proteins were upregulated and 31 downregulated significantly in ‘Laura’, while in ‘Early Bird’, 5 were upregulated and 8 downregulated significantly in high-salt treatment compared to control. We are currently conducting similar analyses in lettuce cultivars from other lettuce types to identify physiological traits, proteins and genes that are important for salinity tolerance.