Salinity Tolerance of Lettuce Cultivars in Controlled Environment

Abstract

The specific objectives of this study were to determine the effects of increasing salinity on growth, crop quality, and physiological parameters of different lettuce (Lactuca sativa L.) cultivars grown either in a hydroponic system or in soil and subjected to irrigation water of varying salinity levels. Two trials were conducted in winter 2016 and summer 2016 in a greenhouse using a hydroponic system for the cultivation of three lettuce cultivars. 'Romaine del Sol', Leaf Lettuce 'Bergams Green' and 'Green Leaf Lettuce' were exposed to irrigation water with increasing salinity (2.1, 3.6, 5.1, and 6.6 dS/m) by supplementing the nutrient solution (2.1 dS/m) with a combination of 2:1 NaCl and CaCl2. Lettuce head height, diameter, leaf number, shoot and root dry weight were not impacted by increasing salinity. Similarly, osmotic potential, transpiration and leaf temperature were not affected. However, head fresh weight and water content were reduced at the higher salinity levels compared to the control in the second trial. A third greenhouse trial was conducted in winter 2017 with 'Romaine del Sol' and 'Green Leaf Lettuce' cultivars grown in a hydroponics system or in containers with soil to determine tolerance to increasing salinity in different substrates. Head height, diameter, and shoot dry mass decreased at the two highest salinity levels at the final harvest. When plants were smaller, salinity had no effect on these variables. Fresh weight, water content, and leaf number decreased with increasing salinity at final harvest for both cultivars however, osmotic potential of both cultivars was not affected by salinity or substrate throughout the study. An informal taste test found that the leaves from the two highest levels of salinity from both cultivars were inedible because of a salty and bitter taste. Mineral concentration of sodium and chloride in ‘Romaine’ and 'Green Leaf Lettuce' increased as salinity levels increased, and plants of both cultivars grown in soil had greater concentrations of both elements when compared to hydroponics. 'Romaine' and 'Green Leaf Lettuce' are more tolerant to salinity than previously reported in other lettuce cultivars, and the physiological variables measured showed little changes in response to increasing salinity. Although lettuce grown at 5.1 dS/m and 6.6 dS/m was marginally acceptable by size standards, the lack of head formation in ‘Romaine del Sol’, and the unfavorable taste of both cultivars would render them unmarketable.