Phenomic and physiological analysis of salinity effects on lettuce

Authors: Adhikari, Neil; Simko, Ivan; Mou, Beiquan

Submitted to: Sensors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2019
Publication Date: 11/5/2019
Publication URL: https://handle.nal.usda.gov/10113/6811085
Citation: Adhikari, N.D., Simko, I., Mou, B. 2019. Phenomic and physiological analysis of salinity effects on lettuce. Sensors. 19(21):4814. https://doi.org/10.3390/s19214814.
DOI: https://doi.org/10.3390/s19214814

Interpretive Summary: Rising salinity in soil and irrigation water is a significant global challenge to agriculture, resulting in income losses amounting to billions of dollars each year. Many lettuce growing regions face challenges related to increasing salinity. Lettuce is a salinity sensitive crop, and loses biomass, nutrients and antioxidants when grown under high salinity. In saline conditions, lettuce leaves exhibit leaf burn and die prematurely. We studied physiological traits of lettuce that may be important in tolerance to salinity. We compared reactions to salinity by five salinity-tolerant and five salinity-sensitive varieties of crisphead, butterhead, romaine, leaf-type, based on results from a previous study, and their wild relative, Lactuca serriola L.. Choosing extremes of sensitive and tolerant lettuce genotypes would let us study the physiological differences in response to salinity in detail. Seedlings were germinated in distilled water to ensure synchronized germination. Uniform, two-day-old seedlings were transplanted into 4-inch pots held in trays. Nutrient solution either without or with sodium chloride (control and salinity treatments, respectively) was applied to the plants by reverse irrigation to prevent injury to leaves. Salinity was gradually increased from 0mM/ 0mM sodium chloride (NaCl)/ Calcium chloride (CaCl2) at the time of transplanting, to 30mM/ 15mM NaCl/ CaCl2 at the beginning of week 3, and maintained until harvest. Plants were grown in growth chambers under 24-hour light of constant fluence, 20°C and 50-70% relative humidity. High-throughput phenomics and lab analyses were used to evaluate the health, photosynthetic capacity and overall performance of the plants. A broad range of reaction to salinity was observed in the 10 cultivars/accessions, with zero to 64% reduction in leaf area and 16% to 67% reduction in fresh weight compared to growth in control conditions. Chlorophyll content increased 5% to 21% in salinity in all cultivated genotypes, while it decreased 5% to 17% in both wild lettuce accessions. No significant change in Photosynthetic CO2 assimilation was observed in any genotype. However, fitness of the photosynthetic apparatus was significantly reduced by salinity in the most sensitive genotypes. By increasing chlorophyll levels under salinity, lettuce plants may have developed an adaptation mechanism to maintain photosynthesis while under salinity stress, and will be interesting to study in the future.

Technical Abstract: Salinity is a rising concern in many lettuce-growing regions. Lettuce (Lactuca sativa L.) is sensitive to salinity, which reduces plant biomass, and causes leaf burn and early senescence. We sought to identify physiological traits important in salt tolerance that allows lettuce adaptation to high salinity while maintaining its productivity. Based on previous salinity tolerance studies, one sensitive and one tolerant genotype each was selected from crisphead, butterhead, and romaine, as well as leaf types of cultivated lettuce and its wild relative, L. serriola L. Physiological parameters were measured four weeks after transplanting two-day old seedlings into 350 mL volume pots filled with sand, hydrated with Hoagland nutrient solution and grown in a growth chamber. Salinity treatment consisted of gradually increasing concentrations of NaCl and CaCl2 from 0 mM/0 mM at the time of transplanting, to 30 mM/15 mM at the beginning of week three, and maintaining it until harvest. Across the 10 genotypes, leaf area and fresh weight decreased 0–64% and 16–67%, respectively, under salinity compared to the control. Salinity stress increased the chlorophyll index by 4–26% in the cultivated genotypes, while decreasing it by 5–14% in the two wild accessions. Tolerant lines less affected by elevated salinity were characterized by high values of the chlorophyll fluorescence parameters Fv/Fm and instantaneous photosystem II quantum yield (QY), and lower leaf transpiration.