Enhancing Freeze Tolerance of Field-grown Lettuce with Salicylic Acid, Ascorbic Acid, and Calcium Chloride

With the climate continuing to change, specialty crop growers are particularly at risk for economic loss caused by erratic weather conditions, excess heat and rain, and frost damage. Although tolerant to cold, lettuce (Lactuca sativa) is a major horticultural crop at risk for exposure to freezing temperatures in late fall or early spring in the Midwest. Exogenous applications of salicylic acid, ascorbic acid, and calcium chloride have been shown to improve abiotic stress in plants, particularly with freezing tolerance. This research investigated the effects of the exogenous application of salicylic acid, ascorbic acid, and calcium chloride, in varying concentrations, on field-grown lettuce. The study was conducted at the Iowa State University Horticulture Research Station, Ames, IA, USA. Weekly applications of salicylic acid (0.5 and 1.0 mM), ascorbic acid (0.5 and 1.0 mM), and calcium chloride (10 and 20 mM) were applied until plants were a marketable size. A control treatment (no spray) was also included. Data regarding plant size, yield, leaf area, leaf number, plant dry weight, plant nutrient analysis, and freeze tolerance were collected. Freeze tolerance assessments were conducted through laboratory-simulated freeze events and the evaluation of natural in-field freeze events. Freeze injury was quantified through the electrolyte leakage method. Marketable weights in 2020 and 2021 were statistically similar, except for 1.0 mM salicylic acid in 2020, which showed a significantly lower marketable weight and head diameter. Trends during both years showed that stress protectant applications had the most effective freeze protection at −12 °C in laboratory-simulated freeze events. Calcium chloride at 20 mM had the highest protection at −12 °C, with 13.3% and 24.0% less injury compared with the control in 2020 and 2021, respectively. Salicylic acid 1.0 mM at −12 °C had 4.7% and 23.7% less injury compared with control during these two years, respectively. Ascorbic acid 1.0 mM at −12 °C also showed 6.8% and 9.5% less injury than the control in 2020 and 2021, respectively. Similar trends were observed after in-field freezing events, with 20 mM calcium chloride providing the highest protection against frost in 2020 and 2021. These findings advance the understanding of the capabilities of stress protectants on lettuce as chemical primers after freezing events. This research highlights the benefits of ascorbic acid, salicylic acid, and calcium chloride as stress protectants and encourages future research and further exploration of concentrations, methods of application, and timing of application of products.